On the Origin of Species

CHAPTER 8.
HYBRIDISM.
Distinction between the sterility of first crosses and of hybrids. Sterility various in degree, not universal,
affected by close interbreeding, removed by domestication. Laws governing the sterility of hybrids. Sterility
not a special endowment, but incidental on other differences. Causes of the sterility of first crosses and of
hybrids. Parallelism between the effects of changed conditions of life and crossing. Fertility of varieties when
crossed and of their mongrel offspring not universal. Hybrids and mongrels compared independently of their
fertility. Summary.
The view generally entertained by naturalists is that species, when intercrossed, have been specially endowed
with the quality of sterility, in order to prevent the confusion of all organic forms. This view certainly seems
at first probable, for species within the same country could hardly have kept distinct had they been capable of
crossing freely. The importance of the fact that hybrids are very generally sterile, has, I think, been much
underrated by some late writers. On the theory of natural selection the case is especially important, inasmuch
as the sterility of hybrids could not possibly be of any advantage to them, and therefore could not have been
acquired by the continued preservation of successive profitable degrees of sterility. I hope, however, to be
able to show that sterility is not a specially acquired or endowed quality, but is incidental on other acquired
differences.
In treating this subject, two classes of facts, to a large extent fundamentally different, have generally been
confounded together; namely, the sterility of two species when first crossed, and the sterility of the hybrids
produced from them.
Pure species have of course their organs of reproduction in a perfect condition, yet when intercrossed they
produce either few or no offspring. Hybrids, on the other hand, have their reproductive organs functionally
impotent, as may be clearly seen in the state of the male element in both plants and animals; though the organs
themselves are perfect in structure, as far as the microscope reveals. In the first case the two sexual elements
which go to form the embryo are perfect; in the second case they are either not at all developed, or are
imperfectly developed. This distinction is important, when the cause of the sterility, which is common to the
two cases, has to be considered. The distinction has probably been slurred over, owing to the sterility in both
cases being looked on as a special endowment, beyond the province of our reasoning powers.
The fertility of varieties, that is of the forms known or believed to have descended from common parents,
when intercrossed, and likewise the fertility of their mongrel offspring, is, on my theory, of equal importance
with the sterility of species; for it seems to make a broad and clear distinction between varieties and species.
First, for the sterility of species when crossed and of their hybrid offspring. It is impossible to study the
several memoirs and works of those two conscientious and admirable observers, Kolreuter and Gartner, who
almost devoted their lives to this subject, without being deeply impressed with the high generality of some
degree of sterility. Kolreuter makes the rule universal; but then he cuts the knot, for in ten cases in which he
found two forms, considered by most authors as distinct species, quite fertile together, he unhesitatingly ranks
them as varieties. Gartner, also, makes the rule equally universal; and he disputes the entire fertility of
Kolreuter's ten cases. But in these and in many other cases, Gartner is obliged carefully to count the seeds, in
order to show that there is any degree of sterility. He always compares the maximum number of seeds
produced by two species when crossed and by their hybrid offspring, with the average number produced by
both pure parent-species in a state of nature. But a serious cause of error seems to me to be here introduced: a
plant to be hybridised must be castrated, and, what is often more important, must be secluded in order to
prevent pollen being brought to it by insects from other plants. Nearly all the plants experimentised on by
Gartner were potted, and apparently were kept in a chamber in his house. That these processes are often
injurious to the fertility of a plant cannot be doubted; for Gartner gives in his table about a score of cases of
plants which he castrated, and artificially fertilised with their own pollen, and (excluding all cases such as the
Leguminosae, in which there is an acknowledged difficulty in the manipulation) half of these twenty plants
had their fertility in some degree impaired. Moreover, as Gartner during several years repeatedly crossed the
primrose and cowslip, which we have such good reason to believe to be varieties, and only once or twice
succeeded in getting fertile seed; as he found the common red and blue pimpernels (Anagallis arvensis and
coerulea), which the best botanists rank as varieties, absolutely sterile together; and as he came to the same
conclusion in several other analogous cases; it seems to me that we may well be permitted to doubt whether
many other species are really so sterile, when intercrossed, as Gartner believes.
It is certain, on the one hand, that the sterility of various species when crossed is so different in degree and
graduates away so insensibly, and, on the other hand, that the fertility of pure species is so easily affected by
various circumstances, that for all practical purposes it is most difficult to say where perfect fertility ends and
sterility begins. I think no better evidence of this can be required than that the two most experienced observers
who have ever lived, namely, Kolreuter and Gartner, should have arrived at diametrically opposite
conclusions in regard to the very same species. It is also most instructive to compare--but I have not space
here to enter on details--the evidence advanced by our best botanists on the question whether certain doubtful
forms should be ranked as species or varieties, with the evidence from fertility adduced by different
hybridisers, or by the same author, from experiments made during different years. It can thus be shown that
neither sterility nor fertility affords any clear distinction between species and varieties; but that the evidence
from this source graduates away, and is doubtful in the same degree as is the evidence derived from other
constitutional and structural differences.
In regard to the sterility of hybrids in successive generations; though Gartner was enabled to rear some
hybrids, carefully guarding them from a cross with either pure parent, for six or seven, and in one case for ten
generations, yet he asserts positively that their fertility never increased, but generally greatly decreased. I do
not doubt that this is usually the case, and that the fertility often suddenly decreases in the first few
generations. Nevertheless I believe that in all these experiments the fertility has been diminished by an
independent cause, namely, from close interbreeding. I have collected so large a body of facts, showing that
close interbreeding lessens fertility, and, on the other hand, that an occasional cross with a distinct individual
or variety increases fertility, that I cannot doubt the correctness of this almost universal belief amongst
breeders. Hybrids are seldom raised by experimentalists in great numbers; and as the parent-species, or other
allied hybrids, generally grow in the same garden, the visits of insects must be carefully prevented during the
flowering season: hence hybrids will generally be fertilised during each generation by their own individual
pollen; and I am convinced that this would be injurious to their fertility, already lessened by their hybrid
origin. I am strengthened in this conviction by a remarkable statement repeatedly made by Gartner, namely,
that if even the less fertile hybrids be artificially fertilised with hybrid pollen of the same kind, their fertility,
notwithstanding the frequent ill effects of manipulation, sometimes decidedly increases, and goes on
increasing. Now, in artificial fertilisation pollen is as often taken by chance (as I know from my own
experience) from the anthers of another flower, as from the anthers of the flower itself which is to be
fertilised; so that a cross between two flowers, though probably on the same plant, would be thus effected.
Moreover, whenever complicated experiments are in progress, so careful an observer as Gartner would have
castrated his hybrids, and this would have insured in each generation a cross with the pollen from a distinct
flower, either from the same plant or from another plant of the same hybrid nature. And thus, the strange fact
of the increase of fertility in the successive generations of ARTIFICIALLY FERTILISED hybrids may, I
believe, be accounted for by close interbreeding having been avoided.
Now let us turn to the results arrived at by the third most experienced hybridiser, namely, the Honourable and
Reverend W. Herbert. He is as emphatic in his conclusion that some hybrids are perfectly fertile--as fertile as
the pure parent-species--as are Kolreuter and Gartner that some degree of sterility between distinct species is a
universal law of nature. He experimentised on some of the very same species as did Gartner. The difference in
their results may, I think, be in part accounted for by Herbert's great horticultural skill, and by his having
hothouses at his command. Of his many important statements I will here give only a single one as an example,
namely, that "every ovule in a pod of Crinum capense fertilised by C. revolutum produced a plant, which (he
says) I never saw to occur in a case of its natural fecundation." So that we here have perfect, or even more
than commonly perfect, fertility in a first cross between two distinct species.
This case of the Crinum leads me to refer to a most singular fact, namely, that there are individual plants, as
with certain species of Lobelia, and with all the species of the genus Hippeastrum, which can be far more
easily fertilised by the pollen of another and distinct species, than by their own pollen. For these plants have
been found to yield seed to the pollen of a distinct species, though quite sterile with their own pollen,
notwithstanding that their own pollen was found to be perfectly good, for it fertilised distinct species. So that
certain individual plants and all the individuals of certain species can actually be hybridised much more
readily than they can be self-fertilised! For instance, a bulb of Hippeastrum aulicum produced four flowers;
three were fertilised by Herbert with their own pollen, and the fourth was subsequently fertilised by the pollen
of a compound hybrid descended from three other and distinct species: the result was that "the ovaries of the
three first flowers soon ceased to grow, and after a few days perished entirely, whereas the pod impregnated
by the pollen of the hybrid made vigorous growth and rapid progress to maturity, and bore good seed, which
vegetated freely." In a letter to me, in 1839, Mr. Herbert told me that he had then tried the experiment during
five years, and he continued to try it during several subsequent years, and always with the same result. This
result has, also, been confirmed by other observers in the case of Hippeastrum with its sub-genera, and in the
case of some other genera, as Lobelia, Passiflora and Verbascum. Although the plants in these experiments
appeared perfectly healthy, and although both the ovules and pollen of the same flower were perfectly good
with respect to other species, yet as they were functionally imperfect in their mutual self-action, we must infer
that the plants were in an unnatural state. Nevertheless these facts show on what slight and mysterious causes
the lesser or greater fertility of species when crossed, in comparison with the same species when
self-fertilised, sometimes depends.
The practical experiments of horticulturists, though not made with scientific precision, deserve some notice. It
is notorious in how complicated a manner the species of Pelargonium, Fuchsia, Calceolaria, Petunia,
Rhododendron, etc., have been crossed, yet many of these hybrids seed freely. For instance, Herbert asserts
that a hybrid from Calceolaria integrifolia and plantaginea, species most widely dissimilar in general habit,
"reproduced itself as perfectly as if it had been a natural species from the mountains of Chile." I have taken
some pains to ascertain the degree of fertility of some of the complex crosses of Rhododendrons, and I am
assured that many of them are perfectly fertile. Mr. C. Noble, for instance, informs me that he raises stocks for
grafting from a hybrid between Rhododendron Ponticum and Catawbiense, and that this hybrid "seeds as
freely as it is possible to imagine." Had hybrids, when fairly treated, gone on decreasing in fertility in each
successive generation, as Gartner believes to be the case, the fact would have been notorious to nurserymen.
Horticulturists raise large beds of the same hybrids, and such alone are fairly treated, for by insect agency the
several individuals of the same hybrid variety are allowed to freely cross with each other, and the injurious
influence of close interbreeding is thus prevented. Any one may readily convince himself of the efficiency of
insect-agency by examining the flowers of the more sterile kinds of hybrid rhododendrons, which produce no
pollen, for he will find on their stigmas plenty of pollen brought from other flowers.
In regard to animals, much fewer experiments have been carefully tried than with plants. If our systematic
arrangements can be trusted, that is if the genera of animals are as distinct from each other, as are the genera
of plants, then we may infer that animals more widely separated in the scale of nature can be more easily
crossed than in the case of plants; but the hybrids themselves are, I think, more sterile. I doubt whether any
case of a perfectly fertile hybrid animal can be considered as thoroughly well authenticated. It should,
however, be borne in mind that, owing to few animals breeding freely under confinement, few experiments
have been fairly tried: for instance, the canary-bird has been crossed with nine other finches, but as not one of
these nine species breeds freely in confinement, we have no right to expect that the first crosses between them
and the canary, or that their hybrids, should be perfectly fertile. Again, with respect to the fertility in
successive generations of the more fertile hybrid animals, I hardly know of an instance in which two families
of the same hybrid have been raised at the same time from different parents, so as to avoid the ill effects of
close interbreeding. On the contrary, brothers and sisters have usually been crossed in each successive
generation, in opposition to the constantly repeated admonition of every breeder. And in this case, it is not at
all surprising that the inherent sterility in the hybrids should have gone on increasing. If we were to act thus,
and pair brothers and sisters in the case of any pure animal, which from any cause had the least tendency to
sterility, the breed would assuredly be lost in a very few generations.
Although I do not know of any thoroughly well-authenticated cases of perfectly fertile hybrid animals, I have
some reason to believe that the hybrids from Cervulus vaginalis and Reevesii, and from Phasianus colchicus
with P. torquatus and with P. versicolor are perfectly fertile. The hybrids from the common and Chinese geese
(A. cygnoides), species which are so different that they are generally ranked in distinct genera, have often
bred in this country with either pure parent, and in one single instance they have bred inter se. This was
effected by Mr. Eyton, who raised two hybrids from the same parents but from different hatches; and from
these two birds he raised no less than eight hybrids (grandchildren of the pure geese) from one nest. In India,
however, these cross-bred geese must be far more fertile; for I am assured by two eminently capable judges,
namely Mr. Blyth and Capt. Hutton, that whole flocks of these crossed geese are kept in various parts of the
country; and as they are kept for profit, where neither pure parent-species exists, they must certainly be highly
fertile.
A doctrine which originated with Pallas, has been largely accepted by modern naturalists; namely, that most
of our domestic animals have descended from two or more aboriginal species, since commingled by
intercrossing. On this view, the aboriginal species must either at first have produced quite fertile hybrids, or
the hybrids must have become in subsequent generations quite fertile under domestication. This latter
alternative seems to me the most probable, and I am inclined to believe in its truth, although it rests on no
direct evidence. I believe, for instance, that our dogs have descended from several wild stocks; yet, with
perhaps the exception of certain indigenous domestic dogs of South America, all are quite fertile together; and
analogy makes me greatly doubt, whether the several aboriginal species would at first have freely bred
together and have produced quite fertile hybrids. So again there is reason to believe that our European and the
humped Indian cattle are quite fertile together; but from facts communicated to me by Mr. Blyth, I think they
must be considered as distinct species. On this view of the origin of many of our domestic animals, we must
either give up the belief of the almost universal sterility of distinct species of animals when crossed; or we
must look at sterility, not as an indelible characteristic, but as one capable of being removed by domestication.
Finally, looking to all the ascertained facts on the intercrossing of plants and animals, it may be concluded
that some degree of sterility, both in first crosses and in hybrids, is an extremely general result; but that it
cannot, under our present state of knowledge, be considered as absolutely universal.
LAWS GOVERNING THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.
We will now consider a little more in detail the circumstances and rules governing the sterility of first crosses
and of hybrids. Our chief object will be to see whether or not the rules indicate that species have specially
been endowed with this quality, in order to prevent their crossing and blending together in utter confusion.
The following rules and conclusions are chiefly drawn up from Gartner's admirable work on the hybridisation
of plants. I have taken much pains to ascertain how far the rules apply to animals, and considering how scanty
our knowledge is in regard to hybrid animals, I have been surprised to find how generally the same rules apply
to both kingdoms.
It has been already remarked, that the degree of fertility, both of first crosses and of hybrids, graduates from
zero to perfect fertility. It is surprising in how many curious ways this gradation can be shown to exist; but
only the barest outline of the facts can here be given. When pollen from a plant of one family is placed on the
stigma of a plant of a distinct family, it exerts no more influence than so much inorganic dust. From this
absolute zero of fertility, the pollen of different species of the same genus applied to the stigma of some one
species, yields a perfect gradation in the number of seeds produced, up to nearly complete or even quite
complete fertility; and, as we have seen, in certain abnormal cases, even to an excess of fertility, beyond that
which the plant's own pollen will produce. So in hybrids themselves, there are some which never have
produced, and probably never would produce, even with the pollen of either pure parent, a single fertile seed:
but in some of these cases a first trace of fertility may be detected, by the pollen of one of the pure
parent-species causing the flower of the hybrid to wither earlier than it otherwise would have done; and the
early withering of the flower is well known to be a sign of incipient fertilisation. From this extreme degree of
sterility we have self-fertilised hybrids producing a greater and greater number of seeds up to perfect fertility.
Hybrids from two species which are very difficult to cross, and which rarely produce any offspring, are
generally very sterile; but the parallelism between the difficulty of making a first cross, and the sterility of the
hybrids thus produced--two classes of facts which are generally confounded together--is by no means strict.
There are many cases, in which two pure species can be united with unusual facility, and produce numerous
hybrid-offspring, yet these hybrids are remarkably sterile. On the other hand, there are species which can be
crossed very rarely, or with extreme difficulty, but the hybrids, when at last produced, are very fertile. Even
within the limits of the same genus, for instance in Dianthus, these two opposite cases occur.
The fertility, both of first crosses and of hybrids, is more easily affected by unfavourable conditions, than is
the fertility of pure species. But the degree of fertility is likewise innately variable; for it is not always the
same when the same two species are crossed under the same circumstances, but depends in part upon the
constitution of the individuals which happen to have been chosen for the experiment. So it is with hybrids, for
their degree of fertility is often found to differ greatly in the several individuals raised from seed out of the
same capsule and exposed to exactly the same conditions.
By the term systematic affinity is meant, the resemblance between species in structure and in constitution,
more especially in the structure of parts which are of high physiological importance and which differ little in
the allied species. Now the fertility of first crosses between species, and of the hybrids produced from them, is
largely governed by their systematic affinity. This is clearly shown by hybrids never having been raised
between species ranked by systematists in distinct families; and on the other hand, by very closely allied
species generally uniting with facility. But the correspondence between systematic affinity and the facility of
crossing is by no means strict. A multitude of cases could be given of very closely allied species which will
not unite, or only with extreme difficulty; and on the other hand of very distinct species which unite with the
utmost facility. In the same family there may be a genus, as Dianthus, in which very many species can most
readily be crossed; and another genus, as Silene, in which the most persevering efforts have failed to produce
between extremely close species a single hybrid. Even within the limits of the same genus, we meet with this
same difference; for instance, the many species of Nicotiana have been more largely crossed than the species
of almost any other genus; but Gartner found that N. acuminata, which is not a particularly distinct species,
obstinately failed to fertilise, or to be fertilised by, no less than eight other species of Nicotiana. Very many
analogous facts could be given.
No one has been able to point out what kind, or what amount, of difference in any recognisable character is
sufficient to prevent two species crossing. It can be shown that plants most widely different in habit and
general appearance, and having strongly marked differences in every part of the flower, even in the pollen, in
the fruit, and in the cotyledons, can be crossed. Annual and perennial plants, deciduous and evergreen trees,
plants inhabiting different stations and fitted for extremely different climates, can often be crossed with ease.
By a reciprocal cross between two species, I mean the case, for instance, of a stallion-horse being first crossed
with a female-ass, and then a male-ass with a mare: these two species may then be said to have been
reciprocally crossed. There is often the widest possible difference in the facility of making reciprocal crosses.
Such cases are highly important, for they prove that the capacity in any two species to cross is often
completely independent of their systematic affinity, or of any recognisable difference in their whole
organisation. On the other hand, these cases clearly show that the capacity for crossing is connected with
constitutional differences imperceptible by us, and confined to the reproductive system. This difference in the
result of reciprocal crosses between the same two species was long ago observed by Kolreuter. To give an
instance: Mirabilis jalappa can easily be fertilised by the pollen of M. longiflora, and the hybrids thus
produced are sufficiently fertile; but Kolreuter tried more than two hundred times, during eight following
years, to fertilise reciprocally M. longiflora with the pollen of M. jalappa, and utterly failed. Several other
equally striking cases could be given. Thuret has observed the same fact with certain sea-weeds or Fuci.
Gartner, moreover, found that this difference of facility in making reciprocal crosses is extremely common in
a lesser degree. He has observed it even between forms so closely related (as Matthiola annua and glabra) that
many botanists rank them only as varieties. It is also a remarkable fact, that hybrids raised from reciprocal
crosses, though of course compounded of the very same two species, the one species having first been used as
the father and then as the mother, generally differ in fertility in a small, and occasionally in a high degree.
Several other singular rules could be given from Gartner: for instance, some species have a remarkable power
of crossing with other species; other species of the same genus have a remarkable power of impressing their
likeness on their hybrid offspring; but these two powers do not at all necessarily go together. There are certain
hybrids which instead of having, as is usual, an intermediate character between their two parents, always
closely resemble one of them; and such hybrids, though externally so like one of their pure parent-species, are
with rare exceptions extremely sterile. So again amongst hybrids which are usually intermediate in structure
between their parents, exceptional and abnormal individuals sometimes are born, which closely resemble one
of their pure parents; and these hybrids are almost always utterly sterile, even when the other hybrids raised
from seed from the same capsule have a considerable degree of fertility. These facts show how completely
fertility in the hybrid is independent of its external resemblance to either pure parent.
Considering the several rules now given, which govern the fertility of first crosses and of hybrids, we see that
when forms, which must be considered as good and distinct species, are united, their fertility graduates from
zero to perfect fertility, or even to fertility under certain conditions in excess. That their fertility, besides being
eminently susceptible to favourable and unfavourable conditions, is innately variable. That it is by no means
always the same in degree in the first cross and in the hybrids produced from this cross. That the fertility of
hybrids is not related to the degree in which they resemble in external appearance either parent. And lastly,
that the facility of making a first cross between any two species is not always governed by their systematic
affinity or degree of resemblance to each other. This latter statement is clearly proved by reciprocal crosses
between the same two species, for according as the one species or the other is used as the father or the mother,
there is generally some difference, and occasionally the widest possible difference, in the facility of effecting
an union. The hybrids, moreover, produced from reciprocal crosses often differ in fertility.
Now do these complex and singular rules indicate that species have been endowed with sterility simply to
prevent their becoming confounded in nature? I think not. For why should the sterility be so extremely
different in degree, when various species are crossed, all of which we must suppose it would be equally
important to keep from blending together? Why should the degree of sterility be innately variable in the
individuals of the same species? Why should some species cross with facility, and yet produce very sterile
hybrids; and other species cross with extreme difficulty, and yet produce fairly fertile hybrids? Why should
there often be so great a difference in the result of a reciprocal cross between the same two species? Why, it
may even be asked, has the production of hybrids been permitted? to grant to species the special power of
producing hybrids, and then to stop their further propagation by different degrees of sterility, not strictly
related to the facility of the first union between their parents, seems to be a strange arrangement.
The foregoing rules and facts, on the other hand, appear to me clearly to indicate that the sterility both of first
crosses and of hybrids is simply incidental or dependent on unknown differences, chiefly in the reproductive
systems, of the species which are crossed. The differences being of so peculiar and limited a nature, that, in
reciprocal crosses between two species the male sexual element of the one will often freely act on the female
sexual element of the other, but not in a reversed direction. It will be advisable to explain a little more fully by
an example what I mean by sterility being incidental on other differences, and not a specially endowed
quality. As the capacity of one plant to be grafted or budded on another is so entirely unimportant for its
welfare in a state of nature, I presume that no one will suppose that this capacity is a SPECIALLY endowed
quality, but will admit that it is incidental on differences in the laws of growth of the two plants. We can
sometimes see the reason why one tree will not take on another, from differences in their rate of growth, in the
hardness of their wood, in the period of the flow or nature of their sap, etc.; but in a multitude of cases we can
assign no reason whatever. Great diversity in the size of two plants, one being woody and the other
herbaceous, one being evergreen and the other deciduous, and adaptation to widely different climates, does
not always prevent the two grafting together. As in hybridisation, so with grafting, the capacity is limited by
systematic affinity, for no one has been able to graft trees together belonging to quite distinct families; and, on
the other hand, closely allied species, and varieties of the same species, can usually, but not invariably, be
grafted with ease. But this capacity, as in hybridisation, is by no means absolutely governed by systematic
affinity. Although many distinct genera within the same family have been grafted together, in other cases
species of the same genus will not take on each other. The pear can be grafted far more readily on the quince,
which is ranked as a distinct genus, than on the apple, which is a member of the same genus. Even different
varieties of the pear take with different degrees of facility on the quince; so do different varieties of the apricot
and peach on certain varieties of the plum.
As Gartner found that there was sometimes an innate difference in different INDIVIDUALS of the same two
species in crossing; so Sagaret believes this to be the case with different individuals of the same two species in
being grafted together. As in reciprocal crosses, the facility of effecting an union is often very far from equal,
so it sometimes is in grafting; the common gooseberry, for instance, cannot be grafted on the currant, whereas
the currant will take, though with difficulty, on the gooseberry.
We have seen that the sterility of hybrids, which have their reproductive organs in an imperfect condition, is a
very different case from the difficulty of uniting two pure species, which have their reproductive organs
perfect; yet these two distinct cases run to a certain extent parallel. Something analogous occurs in grafting;
for Thouin found that three species of Robinia, which seeded freely on their own roots, and which could be
grafted with no great difficulty on another species, when thus grafted were rendered barren. On the other
hand, certain species of Sorbus, when grafted on other species, yielded twice as much fruit as when on their
own roots. We are reminded by this latter fact of the extraordinary case of Hippeastrum, Lobelia, etc., which
seeded much more freely when fertilised with the pollen of distinct species, than when self-fertilised with
their own pollen.
We thus see, that although there is a clear and fundamental difference between the mere adhesion of grafted
stocks, and the union of the male and female elements in the act of reproduction, yet that there is a rude
degree of parallelism in the results of grafting and of crossing distinct species. And as we must look at the
curious and complex laws governing the facility with which trees can be grafted on each other as incidental on
unknown differences in their vegetative systems, so I believe that the still more complex laws governing the
facility of first crosses, are incidental on unknown differences, chiefly in their reproductive systems. These
differences, in both cases, follow to a certain extent, as might have been expected, systematic affinity, by
which every kind of resemblance and dissimilarity between organic beings is attempted to be expressed. The
facts by no means seem to me to indicate that the greater or lesser difficulty of either grafting or crossing
together various species has been a special endowment; although in the case of crossing, the difficulty is as
important for the endurance and stability of specific forms, as in the case of grafting it is unimportant for their
welfare.

Blood Clot Detection

Friday October 18, 2013

Red Blood Cells Image: Renjith Krishnan FreeDigitalPhotos.net

MIT researchers have developed a test to detect blood clots that can sometimes go undetected. Using nanoparticles, they developed a way to detect an important blood clotting factor called thrombin. Blood clot formation is directed by platelets and is necessary to prevent excessive blood loss from broken blood vessels. Blood clots can also form during prolonged periods of inactivity or after taking certain medications. Blood clots can be dangerous if they break free and block vessels in vital organs such as the brain, heart, or lungs.
According to researcher Sangeeta Bhatia, "Some patients are at more risk for clotting, but existing blood tests are not consistently able to detect the formation of new clots." The researchers contend that this technology could be used to screen patients for blood clots and to monitor patients who are at high risk for developing blood clots

Protein May Cause Certain Thyroid Cancers

Medullary Thyroid Cancer Cell Image: UT Southwestern Medical Center

Researchers have identified a specific protein that may be responsible for the development of different types of cancers, including a deadly type of thyroid cancer. UT Southwestern researchers discovered that when the protein Cdk5 is over-expressed in certain cells that secrete hormones, medullary thyroid cancer cells tend to develop. The thyroid is an endocrine gland that regulates various bodily processes including heart rate, body temperature, and metabolism. Medullary thyroid cancer is a form of thyroid cancer that is rare and incurable.
According to researcher Dr. James Bibb, "This research is ongoing, and we are now identifying precisely how Cdk5 causes the growth and spread of these forms of cancer with the goal of discovering new drugs." Cdk5 was previously thought to only exist in the brain. This study revealed that Cdk5 can also be found in specific cells of the thyroid called C cells and that the protein can cause cancer. The researchers are hopeful that this information will lead to new treatments for various forms of endocrine cancers.

On the Origin of Species

CHAPTER 7.
INSTINCT.
Instincts comparable with habits, but different in their origin. Instincts graduated. Aphides and ants. Instincts
variable. Domestic instincts, their origin. Natural instincts of the cuckoo, ostrich, and parasitic bees.
Slave-making ants. Hive-bee, its cell-making instinct. Difficulties on the theory of the Natural Selection of
instincts. Neuter or sterile insects. Summary.
The subject of instinct might have been worked into the previous chapters; but I have thought that it would be
more convenient to treat the subject separately, especially as so wonderful an instinct as that of the hive-bee
making its cells will probably have occurred to many readers, as a difficulty sufficient to overthrow my whole
theory. I must premise, that I have nothing to do with the origin of the primary mental powers, any more than
I have with that of life itself. We are concerned only with the diversities of instinct and of the other mental
qualities of animals within the same class.
I will not attempt any definition of instinct. It would be easy to show that several distinct mental actions are
commonly embraced by this term; but every one understands what is meant, when it is said that instinct
impels the cuckoo to migrate and to lay her eggs in other birds' nests. An action, which we ourselves should
require experience to enable us to perform, when performed by an animal, more especially by a very young
one, without any experience, and when performed by many individuals in the same way, without their
knowing for what purpose it is performed, is usually said to be instinctive. But I could show that none of these
characters of instinct are universal. A little dose, as Pierre Huber expresses it, of judgment or reason, often
comes into play, even in animals very low in the scale of nature.
Frederick Cuvier and several of the older metaphysicians have compared instinct with habit. This comparison
gives, I think, a remarkably accurate notion of the frame of mind under which an instinctive action is
performed, but not of its origin. How unconsciously many habitual actions are performed, indeed not rarely in
direct opposition to our conscious will! yet they may be modified by the will or reason. Habits easily become
associated with other habits, and with certain periods of time and states of the body. When once acquired, they
often remain constant throughout life. Several other points of resemblance between instincts and habits could
be pointed out. As in repeating a well-known song, so in instincts, one action follows another by a sort of
rhythm; if a person be interrupted in a song, or in repeating anything by rote, he is generally forced to go back
to recover the habitual train of thought: so P. Huber found it was with a caterpillar, which makes a very
complicated hammock; for if he took a caterpillar which had completed its hammock up to, say, the sixth
stage of construction, and put it into a hammock completed up only to the third stage, the caterpillar simply
re-performed the fourth, fifth, and sixth stages of construction. If, however, a caterpillar were taken out of a
hammock made up, for instance, to the third stage, and were put into one finished up to the sixth stage, so that
much of its work was already done for it, far from feeling the benefit of this, it was much embarrassed, and, in
order to complete its hammock, seemed forced to start from the third stage, where it had left off, and thus tried
to complete the already finished work. If we suppose any habitual action to become inherited--and I think it
can be shown that this does sometimes happen--then the resemblance between what originally was a habit and
an instinct becomes so close as not to be distinguished. If Mozart, instead of playing the pianoforte at three
years old with wonderfully little practice, had played a tune with no practice at all, he might truly be said to
have done so instinctively. But it would be the most serious error to suppose that the greater number of
instincts have been acquired by habit in one generation, and then transmitted by inheritance to succeeding
generations. It can be clearly shown that the most wonderful instincts with which we are acquainted, namely,
those of the hive-bee and of many ants, could not possibly have been thus acquired.
It will be universally admitted that instincts are as important as corporeal structure for the welfare of each
species, under its present conditions of life. Under changed conditions of life, it is at least possible that slight
modifications of instinct might be profitable to a species; and if it can be shown that instincts do vary ever so
little, then I can see no difficulty in natural selection preserving and continually accumulating variations of
instinct to any extent that may be profitable. It is thus, as I believe, that all the most complex and wonderful
instincts have originated. As modifications of corporeal structure arise from, and are increased by, use or
habit, and are diminished or lost by disuse, so I do not doubt it has been with instincts. But I believe that the
effects of habit are of quite subordinate importance to the effects of the natural selection of what may be
called accidental variations of instincts;--that is of variations produced by the same unknown causes which
produce slight deviations of bodily structure.
No complex instinct can possibly be produced through natural selection, except by the slow and gradual
accumulation of numerous, slight, yet profitable, variations. Hence, as in the case of corporeal structures, we
ought to find in nature, not the actual transitional gradations by which each complex instinct has been
acquired--for these could be found only in the lineal ancestors of each species--but we ought to find in the
collateral lines of descent some evidence of such gradations; or we ought at least to be able to show that
gradations of some kind are possible; and this we certainly can do. I have been surprised to find, making
allowance for the instincts of animals having been but little observed except in Europe and North America,
and for no instinct being known amongst extinct species, how very generally gradations, leading to the most
complex instincts, can be discovered. The canon of "Natura non facit saltum" applies with almost equal force
to instincts as to bodily organs. Changes of instinct may sometimes be facilitated by the same species having
different instincts at different periods of life, or at different seasons of the year, or when placed under different
circumstances, etc.; in which case either one or the other instinct might be preserved by natural selection. And
such instances of diversity of instinct in the same species can be shown to occur in nature.
Again as in the case of corporeal structure, and conformably with my theory, the instinct of each species is
good for itself, but has never, as far as we can judge, been produced for the exclusive good of others. One of
the strongest instances of an animal apparently performing an action for the sole good of another, with which I
am acquainted, is that of aphides voluntarily yielding their sweet excretion to ants: that they do so voluntarily,
the following facts show. I removed all the ants from a group of about a dozen aphides on a dock-plant, and
prevented their attendance during several hours. After this interval, I felt sure that the aphides would want to
excrete. I watched them for some time through a lens, but not one excreted; I then tickled and stroked them
with a hair in the same manner, as well as I could, as the ants do with their antennae; but not one excreted.
Afterwards I allowed an ant to visit them, and it immediately seemed, by its eager way of running about, to be
well aware what a rich flock it had discovered; it then began to play with its antennae on the abdomen first of
one aphis and then of another; and each aphis, as soon as it felt the antennae, immediately lifted up its
abdomen and excreted a limpid drop of sweet juice, which was eagerly devoured by the ant. Even the quite
young aphides behaved in this manner, showing that the action was instinctive, and not the result of
experience. But as the excretion is extremely viscid, it is probably a convenience to the aphides to have it
removed; and therefore probably the aphides do not instinctively excrete for the sole good of the ants.
Although I do not believe that any animal in the world performs an action for the exclusive good of another of
a distinct species, yet each species tries to take advantage of the instincts of others, as each takes advantage of
the weaker bodily structure of others. So again, in some few cases, certain instincts cannot be considered as
absolutely perfect; but as details on this and other such points are not indispensable, they may be here passed
over.
As some degree of variation in instincts under a state of nature, and the inheritance of such variations, are
indispensable for the action of natural selection, as many instances as possible ought to have been here given;
but want of space prevents me. I can only assert, that instincts certainly do vary--for instance, the migratory
instinct, both in extent and direction, and in its total loss. So it is with the nests of birds, which vary partly in
dependence on the situations chosen, and on the nature and temperature of the country inhabited, but often
from causes wholly unknown to us: Audubon has given several remarkable cases of differences in nests of the
same species in the northern and southern United States. Fear of any particular enemy is certainly an
instinctive quality, as may be seen in nestling birds, though it is strengthened by experience, and by the sight
of fear of the same enemy in other animals. But fear of man is slowly acquired, as I have elsewhere shown, by
various animals inhabiting desert islands; and we may see an instance of this, even in England, in the greater
wildness of all our large birds than of our small birds; for the large birds have been most persecuted by man.
We may safely attribute the greater wildness of our large birds to this cause; for in uninhabited islands large
birds are not more fearful than small; and the magpie, so wary in England, is tame in Norway, as is the hooded
crow in Egypt.
That the general disposition of individuals of the same species, born in a state of nature, is extremely
diversified, can be shown by a multitude of facts. Several cases also, could be given, of occasional and strange
habits in certain species, which might, if advantageous to the species, give rise, through natural selection, to
quite new instincts. But I am well aware that these general statements, without facts given in detail, can
produce but a feeble effect on the reader's mind. I can only repeat my assurance, that I do not speak without
good evidence.
The possibility, or even probability, of inherited variations of instinct in a state of nature will be strengthened
by briefly considering a few cases under domestication. We shall thus also be enabled to see the respective
parts which habit and the selection of so-called accidental variations have played in modifying the mental
qualities of our domestic animals. A number of curious and authentic instances could be given of the
inheritance of all shades of disposition and tastes, and likewise of the oddest tricks, associated with certain
frames of mind or periods of time. But let us look to the familiar case of the several breeds of dogs: it cannot
be doubted that young pointers (I have myself seen a striking instance) will sometimes point and even back
other dogs the very first time that they are taken out; retrieving is certainly in some degree inherited by
retrievers; and a tendency to run round, instead of at, a flock of sheep, by shepherd-dogs. I cannot see that
these actions, performed without experience by the young, and in nearly the same manner by each individual,
performed with eager delight by each breed, and without the end being known,--for the young pointer can no
more know that he points to aid his master, than the white butterfly knows why she lays her eggs on the leaf
of the cabbage,--I cannot see that these actions differ essentially from true instincts. If we were to see one kind
of wolf, when young and without any training, as soon as it scented its prey, stand motionless like a statue,
and then slowly crawl forward with a peculiar gait; and another kind of wolf rushing round, instead of at, a
herd of deer, and driving them to a distant point, we should assuredly call these actions instinctive. Domestic
instincts, as they may be called, are certainly far less fixed or invariable than natural instincts; but they have
been acted on by far less rigorous selection, and have been transmitted for an incomparably shorter period,
under less fixed conditions of life.
How strongly these domestic instincts, habits, and dispositions are inherited, and how curiously they become
mingled, is well shown when different breeds of dogs are crossed. Thus it is known that a cross with a
bull-dog has affected for many generations the courage and obstinacy of greyhounds; and a cross with a
greyhound has given to a whole family of shepherd-dogs a tendency to hunt hares. These domestic instincts,
when thus tested by crossing, resemble natural instincts, which in a like manner become curiously blended
together, and for a long period exhibit traces of the instincts of either parent: for example, Le Roy describes a
dog, whose great-grandfather was a wolf, and this dog showed a trace of its wild parentage only in one way,
by not coming in a straight line to his master when called.
Domestic instincts are sometimes spoken of as actions which have become inherited solely from
long-continued and compulsory habit, but this, I think, is not true. No one would ever have thought of
teaching, or probably could have taught, the tumbler-pigeon to tumble,--an action which, as I have witnessed,
is performed by young birds, that have never seen a pigeon tumble. We may believe that some one pigeon
showed a slight tendency to this strange habit, and that the long-continued selection of the best individuals in
successive generations made tumblers what they now are; and near Glasgow there are house-tumblers, as I
hear from Mr. Brent, which cannot fly eighteen inches high without going head over heels. It may be doubted
whether any one would have thought of training a dog to point, had not some one dog naturally shown a
tendency in this line; and this is known occasionally to happen, as I once saw in a pure terrier. When the first
tendency was once displayed, methodical selection and the inherited effects of compulsory training in each
successive generation would soon complete the work; and unconscious selection is still at work, as each man
tries to procure, without intending to improve the breed, dogs which will stand and hunt best. On the other
hand, habit alone in some cases has sufficed; no animal is more difficult to tame than the young of the wild
rabbit; scarcely any animal is tamer than the young of the tame rabbit; but I do not suppose that domestic
rabbits have ever been selected for tameness; and I presume that we must attribute the whole of the inherited
change from extreme wildness to extreme tameness, simply to habit and long-continued close confinement.
Natural instincts are lost under domestication: a remarkable instance of this is seen in those breeds of fowls
which very rarely or never become "broody," that is, never wish to sit on their eggs. Familiarity alone
prevents our seeing how universally and largely the minds of our domestic animals have been modified by
domestication. It is scarcely possible to doubt that the love of man has become instinctive in the dog. All
wolves, foxes, jackals, and species of the cat genus, when kept tame, are most eager to attack poultry, sheep,
and pigs; and this tendency has been found incurable in dogs which have been brought home as puppies from
countries, such as Tierra del Fuego and Australia, where the savages do not keep these domestic animals. How
rarely, on the other hand, do our civilised dogs, even when quite young, require to be taught not to attack
poultry, sheep, and pigs! No doubt they occasionally do make an attack, and are then beaten; and if not cured,
they are destroyed; so that habit, with some degree of selection, has probably concurred in civilising by
inheritance our dogs. On the other hand, young chickens have lost, wholly by habit, that fear of the dog and
cat which no doubt was originally instinctive in them, in the same way as it is so plainly instinctive in young
pheasants, though reared under a hen. It is not that chickens have lost all fear, but fear only of dogs and cats,
for if the hen gives the danger-chuckle, they will run (more especially young turkeys) from under her, and
conceal themselves in the surrounding grass or thickets; and this is evidently done for the instinctive purpose
of allowing, as we see in wild ground-birds, their mother to fly away. But this instinct retained by our
chickens has become useless under domestication, for the mother-hen has almost lost by disuse the power of
flight.
Hence, we may conclude, that domestic instincts have been acquired and natural instincts have been lost
partly by habit, and partly by man selecting and accumulating during successive generations, peculiar mental
habits and actions, which at first appeared from what we must in our ignorance call an accident. In some cases
compulsory habit alone has sufficed to produce such inherited mental changes; in other cases compulsory
habit has done nothing, and all has been the result of selection, pursued both methodically and unconsciously;
but in most cases, probably, habit and selection have acted together.
We shall, perhaps, best understand how instincts in a state of nature have become modified by selection, by
considering a few cases. I will select only three, out of the several which I shall have to discuss in my future
work,--namely, the instinct which leads the cuckoo to lay her eggs in other birds' nests; the slave-making
instinct of certain ants; and the comb-making power of the hive-bee: these two latter instincts have generally,
and most justly, been ranked by naturalists as the most wonderful of all known instincts.
It is now commonly admitted that the more immediate and final cause of the cuckoo's instinct is, that she lays
her eggs, not daily, but at intervals of two or three days; so that, if she were to make her own nest and sit on
her own eggs, those first laid would have to be left for some time unincubated, or there would be eggs and
young birds of different ages in the same nest. If this were the case, the process of laying and hatching might
be inconveniently long, more especially as she has to migrate at a very early period; and the first hatched
young would probably have to be fed by the male alone. But the American cuckoo is in this predicament; for
she makes her own nest and has eggs and young successively hatched, all at the same time. It has been
asserted that the American cuckoo occasionally lays her eggs in other birds' nests; but I hear on the high
authority of Dr. Brewer, that this is a mistake. Nevertheless, I could give several instances of various birds
which have been known occasionally to lay their eggs in other birds' nests. Now let us suppose that the
ancient progenitor of our European cuckoo had the habits of the American cuckoo; but that occasionally she
laid an egg in another bird's nest. If the old bird profited by this occasional habit, or if the young were made
more vigorous by advantage having been taken of the mistaken maternal instinct of another bird, than by their
own mother's care, encumbered as she can hardly fail to be by having eggs and young of different ages at the
same time; then the old birds or the fostered young would gain an advantage. And analogy would lead me to
believe, that the young thus reared would be apt to follow by inheritance the occasional and aberrant habit of
their mother, and in their turn would be apt to lay their eggs in other birds' nests, and thus be successful in
rearing their young. By a continued process of this nature, I believe that the strange instinct of our cuckoo
could be, and has been, generated. I may add that, according to Dr. Gray and to some other observers, the
European cuckoo has not utterly lost all maternal love and care for her own offspring.
The occasional habit of birds laying their eggs in other birds' nests, either of the same or of a distinct species,
is not very uncommon with the Gallinaceae; and this perhaps explains the origin of a singular instinct in the
allied group of ostriches. For several hen ostriches, at least in the case of the American species, unite and lay
first a few eggs in one nest and then in another; and these are hatched by the males. This instinct may
probably be accounted for by the fact of the hens laying a large number of eggs; but, as in the case of the
cuckoo, at intervals of two or three days. This instinct, however, of the American ostrich has not as yet been
perfected; for a surprising number of eggs lie strewed over the plains, so that in one day's hunting I picked up
no less than twenty lost and wasted eggs.
Many bees are parasitic, and always lay their eggs in the nests of bees of other kinds. This case is more
remarkable than that of the cuckoo; for these bees have not only their instincts but their structure modified in
accordance with their parasitic habits; for they do not possess the pollen-collecting apparatus which would be
necessary if they had to store food for their own young. Some species, likewise, of Sphegidae (wasp-like
insects) are parasitic on other species; and M. Fabre has lately shown good reason for believing that although
the Tachytes nigra generally makes its own burrow and stores it with paralysed prey for its own larvae to feed
on, yet that when this insect finds a burrow already made and stored by another sphex, it takes advantage of
the prize, and becomes for the occasion parasitic. In this case, as with the supposed case of the cuckoo, I can
see no difficulty in natural selection making an occasional habit permanent, if of advantage to the species, and
if the insect whose nest and stored food are thus feloniously appropriated, be not thus exterminated.

On the Origin of Species

CHAPTER 6.
DIFFICULTIES ON THEORY.
Difficulties on the theory of descent with modification. Transitions. Absence or rarity of transitional varieties.
Transitions in habits of life. Diversified habits in the same species. Species with habits widely different from
those of their allies. Organs of extreme perfection. Means of transition. Cases of difficulty. Natura non facit
saltum. Organs of small importance. Organs not in all cases absolutely perfect. The law of Unity of Type and
of the Conditions of Existence embraced by the theory of Natural Selection.
Long before having arrived at this part of my work, a crowd of difficulties will have occurred to the reader.
Some of them are so grave that to this day I can never reflect on them without being staggered; but, to the best
of my judgment, the greater number are only apparent, and those that are real are not, I think, fatal to my
theory.
These difficulties and objections may be classed under the following heads:--
Firstly, why, if species have descended from other species by insensibly fine gradations, do we not
everywhere see innumerable transitional forms? Why is not all nature in confusion instead of the species
being, as we see them, well defined?
Secondly, is it possible that an animal having, for instance, the structure and habits of a bat, could have been
formed by the modification of some animal with wholly different habits? Can we believe that natural selection
could produce, on the one hand, organs of trifling importance, such as the tail of a giraffe, which serves as a
fly-flapper, and, on the other hand, organs of such wonderful structure, as the eye, of which we hardly as yet
fully understand the inimitable perfection?
Thirdly, can instincts be acquired and modified through natural selection? What shall we say to so marvellous
an instinct as that which leads the bee to make cells, which have practically anticipated the discoveries of
profound mathematicians?
Fourthly, how can we account for species, when crossed, being sterile and producing sterile offspring,
whereas, when varieties are crossed, their fertility is unimpaired?
The two first heads shall be here discussed--Instinct and Hybridism in separate chapters.
ON THE ABSENCE OR RARITY OF TRANSITIONAL VARIETIES.
As natural selection acts solely by the preservation of profitable modifications, each new form will tend in a
fully-stocked country to take the place of, and finally to exterminate, its own less improved parent or other
less-favoured forms with which it comes into competition. Thus extinction and natural selection will, as we
have seen, go hand in hand. Hence, if we look at each species as descended from some other unknown form,
both the parent and all the transitional varieties will generally have been exterminated by the very process of
formation and perfection of the new form.
But, as by this theory innumerable transitional forms must have existed, why do we not find them embedded
in countless numbers in the crust of the earth? It will be much more convenient to discuss this question in the
chapter on the Imperfection of the geological record; and I will here only state that I believe the answer
mainly lies in the record being incomparably less perfect than is generally supposed; the imperfection of the
record being chiefly due to organic beings not inhabiting profound depths of the sea, and to their remains
being embedded and preserved to a future age only in masses of sediment sufficiently thick and extensive to
withstand an enormous amount of future degradation; and such fossiliferous masses can be accumulated only
where much sediment is deposited on the shallow bed of the sea, whilst it slowly subsides. These
contingencies will concur only rarely, and after enormously long intervals. Whilst the bed of the sea is
stationary or is rising, or when very little sediment is being deposited, there will be blanks in our geological
history. The crust of the earth is a vast museum; but the natural collections have been made only at intervals
of time immensely remote.
But it may be urged that when several closely-allied species inhabit the same territory we surely ought to find
at the present time many transitional forms. Let us take a simple case: in travelling from north to south over a
continent, we generally meet at successive intervals with closely allied or representative species, evidently
filling nearly the same place in the natural economy of the land. These representative species often meet and
interlock; and as the one becomes rarer and rarer, the other becomes more and more frequent, till the one
replaces the other. But if we compare these species where they intermingle, they are generally as absolutely
distinct from each other in every detail of structure as are specimens taken from the metropolis inhabited by
each. By my theory these allied species have descended from a common parent; and during the process of
modification, each has become adapted to the conditions of life of its own region, and has supplanted and
exterminated its original parent and all the transitional varieties between its past and present states. Hence we
ought not to expect at the present time to meet with numerous transitional varieties in each region, though
they must have existed there, and may be embedded there in a fossil condition. But in the intermediate region,
having intermediate conditions of life, why do we not now find closely-linking intermediate varieties? This
difficulty for a long time quite confounded me. But I think it can be in large part explained.
In the first place we should be extremely cautious in inferring, because an area is now continuous, that it has
been continuous during a long period. Geology would lead us to believe that almost every continent has been
broken up into islands even during the later tertiary periods; and in such islands distinct species might have
been separately formed without the possibility of intermediate varieties existing in the intermediate zones. By
changes in the form of the land and of climate, marine areas now continuous must often have existed within
recent times in a far less continuous and uniform condition than at present. But I will pass over this way of
escaping from the difficulty; for I believe that many perfectly defined species have been formed on strictly
continuous areas; though I do not doubt that the formerly broken condition of areas now continuous has
played an important part in the formation of new species, more especially with freely-crossing and wandering
animals.
In looking at species as they are now distributed over a wide area, we generally find them tolerably numerous
over a large territory, then becoming somewhat abruptly rarer and rarer on the confines, and finally
disappearing. Hence the neutral territory between two representative species is generally narrow in
comparison with the territory proper to each. We see the same fact in ascending mountains, and sometimes it
is quite remarkable how abruptly, as Alph. De Candolle has observed, a common alpine species disappears.
The same fact has been noticed by Forbes in sounding the depths of the sea with the dredge. To those who
look at climate and the physical conditions of life as the all-important elements of distribution, these facts
ought to cause surprise, as climate and height or depth graduate away insensibly. But when we bear in mind
that almost every species, even in its metropolis, would increase immensely in numbers, were it not for other
competing species; that nearly all either prey on or serve as prey for others; in short, that each organic being is
either directly or indirectly related in the most important manner to other organic beings, we must see that the
range of the inhabitants of any country by no means exclusively depends on insensibly changing physical
conditions, but in large part on the presence of other species, on which it depends, or by which it is destroyed,
or with which it comes into competition; and as these species are already defined objects (however they may
have become so), not blending one into another by insensible gradations, the range of any one species,
depending as it does on the range of others, will tend to be sharply defined. Moreover, each species on the
confines of its range, where it exists in lessened numbers, will, during fluctuations in the number of its
enemies or of its prey, or in the seasons, be extremely liable to utter extermination; and thus its geographical
range will come to be still more sharply defined.
If I am right in believing that allied or representative species, when inhabiting a continuous area, are generally
so distributed that each has a wide range, with a comparatively narrow neutral territory between them, in
which they become rather suddenly rarer and rarer; then, as varieties do not essentially differ from species, the
same rule will probably apply to both; and if we in imagination adapt a varying species to a very large area,
we shall have to adapt two varieties to two large areas, and a third variety to a narrow intermediate zone. The
intermediate variety, consequently, will exist in lesser numbers from inhabiting a narrow and lesser area; and
practically, as far as I can make out, this rule holds good with varieties in a state of nature. I have met with
striking instances of the rule in the case of varieties intermediate between well-marked varieties in the genus
Balanus. And it would appear from information given me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston,
that generally when varieties intermediate between two other forms occur, they are much rarer numerically
than the forms which they connect. Now, if we may trust these facts and inferences, and therefore conclude
that varieties linking two other varieties together have generally existed in lesser numbers than the forms
which they connect, then, I think, we can understand why intermediate varieties should not endure for very
long periods;--why as a general rule they should be exterminated and disappear, sooner than the forms which
they originally linked together.
For any form existing in lesser numbers would, as already remarked, run a greater chance of being
exterminated than one existing in large numbers; and in this particular case the intermediate form would be
eminently liable to the inroads of closely allied forms existing on both sides of it. But a far more important
consideration, as I believe, is that, during the process of further modification, by which two varieties are
supposed on my theory to be converted and perfected into two distinct species, the two which exist in larger
numbers from inhabiting larger areas, will have a great advantage over the intermediate variety, which exists
in smaller numbers in a narrow and intermediate zone. For forms existing in larger numbers will always have
a better chance, within any given period, of presenting further favourable variations for natural selection to
seize on, than will the rarer forms which exist in lesser numbers. Hence, the more common forms, in the race
for life, will tend to beat and supplant the less common forms, for these will be more slowly modified and
improved. It is the same principle which, as I believe, accounts for the common species in each country, as
shown in the second chapter, presenting on an average a greater number of well-marked varieties than do the
rarer species. I may illustrate what I mean by supposing three varieties of sheep to be kept, one adapted to an
extensive mountainous region; a second to a comparatively narrow, hilly tract; and a third to wide plains at the
base; and that the inhabitants are all trying with equal steadiness and skill to improve their stocks by selection;
the chances in this case will be strongly in favour of the great holders on the mountains or on the plains
improving their breeds more quickly than the small holders on the intermediate narrow, hilly tract; and
consequently the improved mountain or plain breed will soon take the place of the less improved hill breed;
and thus the two breeds, which originally existed in greater numbers, will come into close contact with each
other, without the interposition of the supplanted, intermediate hill-variety.
To sum up, I believe that species come to be tolerably well-defined objects, and do not at any one period
present an inextricable chaos of varying and intermediate links: firstly, because new varieties are very slowly
formed, for variation is a very slow process, and natural selection can do nothing until favourable variations
chance to occur, and until a place in the natural polity of the country can be better filled by some modification
of some one or more of its inhabitants. And such new places will depend on slow changes of climate, or on
the occasional immigration of new inhabitants, and, probably, in a still more important degree, on some of the
old inhabitants becoming slowly modified, with the new forms thus produced and the old ones acting and
reacting on each other. So that, in any one region and at any one time, we ought only to see a few species
presenting slight modifications of structure in some degree permanent; and this assuredly we do see.
Secondly, areas now continuous must often have existed within the recent period in isolated portions, in
which many forms, more especially amongst the classes which unite for each birth and wander much, may
have separately been rendered sufficiently distinct to rank as representative species. In this case, intermediate
varieties between the several representative species and their common parent, must formerly have existed in
each broken portion of the land, but these links will have been supplanted and exterminated during the process
of natural selection, so that they will no longer exist in a living state.
Thirdly, when two or more varieties have been formed in different portions of a strictly continuous area,
intermediate varieties will, it is probable, at first have been formed in the intermediate zones, but they will
generally have had a short duration. For these intermediate varieties will, from reasons already assigned
(namely from what we know of the actual distribution of closely allied or representative species, and likewise
of acknowledged varieties), exist in the intermediate zones in lesser numbers than the varieties which they
tend to connect. From this cause alone the intermediate varieties will be liable to accidental extermination;
and during the process of further modification through natural selection, they will almost certainly be beaten
and supplanted by the forms which they connect; for these from existing in greater numbers will, in the
aggregate, present more variation, and thus be further improved through natural selection and gain further
advantages.
Lastly, looking not to any one time, but to all time, if my theory be true, numberless intermediate varieties,
linking most closely all the species of the same group together, must assuredly have existed; but the very
process of natural selection constantly tends, as has been so often remarked, to exterminate the parent forms
and the intermediate links. Consequently evidence of their former existence could be found only amongst
fossil remains, which are preserved, as we shall in a future chapter attempt to show, in an extremely imperfect
and intermittent record.
ON THE ORIGIN AND TRANSITIONS OF ORGANIC BEINGS WITH PECULIAR HABITS AND
STRUCTURE.
It has been asked by the opponents of such views as I hold, how, for instance, a land carnivorous animal could
have been converted into one with aquatic habits; for how could the animal in its transitional state have
subsisted? It would be easy to show that within the same group carnivorous animals exist having every
intermediate grade between truly aquatic and strictly terrestrial habits; and as each exists by a struggle for life,
it is clear that each is well adapted in its habits to its place in nature. Look at the Mustela vison of North
America, which has webbed feet and which resembles an otter in its fur, short legs, and form of tail; during
summer this animal dives for and preys on fish, but during the long winter it leaves the frozen waters, and
preys like other polecats on mice and land animals. If a different case had been taken, and it had been asked
how an insectivorous quadruped could possibly have been converted into a flying bat, the question would
have been far more difficult, and I could have given no answer. Yet I think such difficulties have very little
weight.
Here, as on other occasions, I lie under a heavy disadvantage, for out of the many striking cases which I have
collected, I can give only one or two instances of transitional habits and structures in closely allied species of
the same genus; and of diversified habits, either constant or occasional, in the same species. And it seems to
me that nothing less than a long list of such cases is sufficient to lessen the difficulty in any particular case
like that of the bat.
Look at the family of squirrels; here we have the finest gradation from animals with their tails only slightly
flattened, and from others, as Sir J. Richardson has remarked, with the posterior part of their bodies rather
wide and with the skin on their flanks rather full, to the so-called flying squirrels; and flying squirrels have
their limbs and even the base of the tail united by a broad expanse of skin, which serves as a parachute and
allows them to glide through the air to an astonishing distance from tree to tree. We cannot doubt that each
structure is of use to each kind of squirrel in its own country, by enabling it to escape birds or beasts of prey,
or to collect food more quickly, or, as there is reason to believe, by lessening the danger from occasional falls.
But it does not follow from this fact that the structure of each squirrel is the best that it is possible to conceive
under all natural conditions. Let the climate and vegetation change, let other competing rodents or new beasts
of prey immigrate, or old ones become modified, and all analogy would lead us to believe that some at least of
the squirrels would decrease in numbers or become exterminated, unless they also became modified and
improved in structure in a corresponding manner. Therefore, I can see no difficulty, more especially under
changing conditions of life, in the continued preservation of individuals with fuller and fuller
flank-membranes, each modification being useful, each being propagated, until by the accumulated effects of
this process of natural selection, a perfect so-called flying squirrel was produced.
Now look at the Galeopithecus or flying lemur, which formerly was falsely ranked amongst bats. It has an
extremely wide flank-membrane, stretching from the corners of the jaw to the tail, and including the limbs and
the elongated fingers: the flank membrane is, also, furnished with an extensor muscle. Although no graduated
links of structure, fitted for gliding through the air, now connect the Galeopithecus with the other Lemuridae,
yet I can see no difficulty in supposing that such links formerly existed, and that each had been formed by the
same steps as in the case of the less perfectly gliding squirrels; and that each grade of structure had been
useful to its possessor. Nor can I see any insuperable difficulty in further believing it possible that the
membrane-connected fingers and fore-arm of the Galeopithecus might be greatly lengthened by natural
selection; and this, as far as the organs of flight are concerned, would convert it into a bat. In bats which have
the wing-membrane extended from the top of the shoulder to the tail, including the hind-legs, we perhaps see
traces of an apparatus originally constructed for gliding through the air rather than for flight.
If about a dozen genera of birds had become extinct or were unknown, who would have ventured to have
surmised that birds might have existed which used their wings solely as flappers, like the logger-headed duck
(Micropterus of Eyton); as fins in the water and front legs on the land, like the penguin; as sails, like the
ostrich; and functionally for no purpose, like the Apteryx. Yet the structure of each of these birds is good for
it, under the conditions of life to which it is exposed, for each has to live by a struggle; but it is not necessarily
the best possible under all possible conditions. It must not be inferred from these remarks that any of the
grades of wing-structure here alluded to, which perhaps may all have resulted from disuse, indicate the natural
steps by which birds have acquired their perfect power of flight; but they serve, at least, to show what
diversified means of transition are possible.
Seeing that a few members of such water-breathing classes as the Crustacea and Mollusca are adapted to live
on the land, and seeing that we have flying birds and mammals, flying insects of the most diversified types,
and formerly had flying reptiles, it is conceivable that flying-fish, which now glide far through the air, slightly
rising and turning by the aid of their fluttering fins, might have been modified into perfectly winged animals.
If this had been effected, who would have ever imagined that in an early transitional state they had been
inhabitants of the open ocean, and had used their incipient organs of flight exclusively, as far as we know, to
escape being devoured by other fish?
When we see any structure highly perfected for any particular habit, as the wings of a bird for flight, we
should bear in mind that animals displaying early transitional grades of the structure will seldom continue to
exist to the present day, for they will have been supplanted by the very process of perfection through natural
selection. Furthermore, we may conclude that transitional grades between structures fitted for very different
habits of life will rarely have been developed at an early period in great numbers and under many subordinate
forms. Thus, to return to our imaginary illustration of the flying-fish, it does not seem probable that fishes
capable of true flight would have been developed under many subordinate forms, for taking prey of many
kinds in many ways, on the land and in the water, until their organs of flight had come to a high stage of
perfection, so as to have given them a decided advantage over other animals in the battle for life. Hence the
chance of discovering species with transitional grades of structure in a fossil condition will always be less,
from their having existed in lesser numbers, than in the case of species with fully developed structures.
I will now give two or three instances of diversified and of changed habits in the individuals of the same
species. When either case occurs, it would be easy for natural selection to fit the animal, by some modification
of its structure, for its changed habits, or exclusively for one of its several different habits. But it is difficult to
tell, and immaterial for us, whether habits generally change first and structure afterwards; or whether slight
modifications of structure lead to changed habits; both probably often change almost simultaneously. Of cases
of changed habits it will suffice merely to allude to that of the many British insects which now feed on exotic
plants, or exclusively on artificial substances. Of diversified habits innumerable instances could be given: I
have often watched a tyrant flycatcher (Saurophagus sulphuratus) in South America, hovering over one spot
and then proceeding to another, like a kestrel, and at other times standing stationary on the margin of water,
and then dashing like a kingfisher at a fish. In our own country the larger titmouse (Parus major) may be seen
climbing branches, almost like a creeper; it often, like a shrike, kills small birds by blows on the head; and I
have many times seen and heard it hammering the seeds of the yew on a branch, and thus breaking them like a
nuthatch. In North America the black bear was seen by Hearne swimming for hours with widely open mouth,
thus catching, like a whale, insects in the water. Even in so extreme a case as this, if the supply of insects were
constant, and if better adapted competitors did not already exist in the country, I can see no difficulty in a race
of bears being rendered, by natural selection, more and more aquatic in their structure and habits, with larger
and larger mouths, till a creature was produced as monstrous as a whale.
As we sometimes see individuals of a species following habits widely different from those both of their own
species and of the other species of the same genus, we might expect, on my theory, that such individuals
would occasionally have given rise to new species, having anomalous habits, and with their structure either
slightly or considerably modified from that of their proper type. And such instances do occur in nature. Can a
more striking instance of adaptation be given than that of a woodpecker for climbing trees and for seizing
insects in the chinks of the bark? Yet in North America there are woodpeckers which feed largely on fruit, and
others with elongated wings which chase insects on the wing; and on the plains of La Plata, where not a tree
grows, there is a woodpecker, which in every essential part of its organisation, even in its colouring, in the
harsh tone of its voice, and undulatory flight, told me plainly of its close blood-relationship to our common
species; yet it is a woodpecker which never climbs a tree!
Petrels are the most aerial and oceanic of birds, yet in the quiet Sounds of Tierra del Fuego, the Puffinuria
berardi, in its general habits, in its astonishing power of diving, its manner of swimming, and of flying when
unwillingly it takes flight, would be mistaken by any one for an auk or grebe; nevertheless, it is essentially a
petrel, but with many parts of its organisation profoundly modified. On the other hand, the acutest observer by
examining the dead body of the water-ouzel would never have suspected its sub-aquatic habits; yet this
anomalous member of the strictly terrestrial thrush family wholly subsists by diving,--grasping the stones with
its feet and using its wings under water.
He who believes that each being has been created as we now see it, must occasionally have felt surprise when
he has met with an animal having habits and structure not at all in agreement. What can be plainer than that
the webbed feet of ducks and geese are formed for swimming? yet there are upland geese with webbed feet
which rarely or never go near the water; and no one except Audubon has seen the frigate-bird, which has all
its four toes webbed, alight on the surface of the sea. On the other hand, grebes and coots are eminently
aquatic, although their toes are only bordered by membrane. What seems plainer than that the long toes of
grallatores are formed for walking over swamps and floating plants, yet the water-hen is nearly as aquatic as
the coot; and the landrail nearly as terrestrial as the quail or partridge. In such cases, and many others could be
given, habits have changed without a corresponding change of structure. The webbed feet of the upland goose
may be said to have become rudimentary in function, though not in structure. In the frigate-bird, the
deeply-scooped membrane between the toes shows that structure has begun to change.
He who believes in separate and innumerable acts of creation will say, that in these cases it has pleased the
Creator to cause a being of one type to take the place of one of another type; but this seems to me only
restating the fact in dignified language. He who believes in the struggle for existence and in the principle of
natural selection, will acknowledge that every organic being is constantly endeavouring to increase in
numbers; and that if any one being vary ever so little, either in habits or structure, and thus gain an advantage
over some other inhabitant of the country, it will seize on the place of that inhabitant, however different it may
be from its own place. Hence it will cause him no surprise that there should be geese and frigate-birds with
webbed feet, either living on the dry land or most rarely alighting on the water; that there should be long-toed
corncrakes living in meadows instead of in swamps; that there should be woodpeckers where not a tree grows;
that there should be diving thrushes, and petrels with the habits of auks.

On the Origine Of Species

CHAPTER 5.
LAWS OF VARIATION.
Effects of external conditions. Use and disuse, combined with natural selection; organs of flight and of vision.
Acclimatisation. Correlation of growth. Compensation and economy of growth. False correlations. Multiple,
rudimentary, and lowly organised structures variable. Parts developed in an unusual manner are highly
variable: specific characters more variable than generic: secondary sexual characters variable. Species of the
same genus vary in an analogous manner. Reversions to long lost characters. Summary.
I have hitherto sometimes spoken as if the variations--so common and multiform in organic beings under
domestication, and in a lesser degree in those in a state of nature--had been due to chance. This, of course, is a
wholly incorrect expression, but it serves to acknowledge plainly our ignorance of the cause of each particular
variation. Some authors believe it to be as much the function of the reproductive system to produce individual
differences, or very slight deviations of structure, as to make the child like its parents. But the much greater
variability, as well as the greater frequency of monstrosities, under domestication or cultivation, than under
nature, leads me to believe that deviations of structure are in some way due to the nature of the conditions of
life, to which the parents and their more remote ancestors have been exposed during several generations. I
have remarked in the first chapter--but a long catalogue of facts which cannot be here given would be
necessary to show the truth of the remark--that the reproductive system is eminently susceptible to changes in
the conditions of life; and to this system being functionally disturbed in the parents, I chiefly attribute the
varying or plastic condition of the offspring. The male and female sexual elements seem to be affected before
that union takes place which is to form a new being. In the case of "sporting" plants, the bud, which in its
earliest condition does not apparently differ essentially from an ovule, is alone affected. But why, because the
reproductive system is disturbed, this or that part should vary more or less, we are profoundly ignorant.
Nevertheless, we can here and there dimly catch a faint ray of light, and we may feel sure that there must be
some cause for each deviation of structure, however slight.
How much direct effect difference of climate, food, etc., produces on any being is extremely doubtful. My
impression is, that the effect is extremely small in the case of animals, but perhaps rather more in that of
plants. We may, at least, safely conclude that such influences cannot have produced the many striking and
complex co-adaptations of structure between one organic being and another, which we see everywhere
throughout nature. Some little influence may be attributed to climate, food, etc.: thus, E. Forbes speaks
confidently that shells at their southern limit, and when living in shallow water, are more brightly coloured
than those of the same species further north or from greater depths. Gould believes that birds of the same
species are more brightly coloured under a clear atmosphere, than when living on islands or near the coast. So
with insects, Wollaston is convinced that residence near the sea affects their colours. Moquin-Tandon gives a
list of plants which when growing near the sea-shore have their leaves in some degree fleshy, though not
elsewhere fleshy. Several other such cases could be given.
The fact of varieties of one species, when they range into the zone of habitation of other species, often
acquiring in a very slight degree some of the characters of such species, accords with our view that species of
all kinds are only well-marked and permanent varieties. Thus the species of shells which are confined to
tropical and shallow seas are generally brighter-coloured than those confined to cold and deeper seas. The
birds which are confined to continents are, according to Mr. Gould, brighter-coloured than those of islands.
The insect-species confined to sea-coasts, as every collector knows, are often brassy or lurid. Plants which
live exclusively on the sea-side are very apt to have fleshy leaves. He who believes in the creation of each
species, will have to say that this shell, for instance, was created with bright colours for a warm sea; but that
this other shell became bright-coloured by variation when it ranged into warmer or shallower waters.
When a variation is of the slightest use to a being, we cannot tell how much of it to attribute to the
accumulative action of natural selection, and how much to the conditions of life. Thus, it is well known to
furriers that animals of the same species have thicker and better fur the more severe the climate is under which
they have lived; but who can tell how much of this difference may be due to the warmest-clad individuals
having been favoured and preserved during many generations, and how much to the direct action of the severe
climate? for it would appear that climate has some direct action on the hair of our domestic quadrupeds.
Instances could be given of the same variety being produced under conditions of life as different as can well
be conceived; and, on the other hand, of different varieties being produced from the same species under the
same conditions. Such facts show how indirectly the conditions of life must act. Again, innumerable instances
are known to every naturalist of species keeping true, or not varying at all, although living under the most
opposite climates. Such considerations as these incline me to lay very little weight on the direct action of the
conditions of life. Indirectly, as already remarked, they seem to play an important part in affecting the
reproductive system, and in thus inducing variability; and natural selection will then accumulate all profitable
variations, however slight, until they become plainly developed and appreciable by us.
EFFECTS OF USE AND DISUSE.
From the facts alluded to in the first chapter, I think there can be little doubt that use in our domestic animals
strengthens and enlarges certain parts, and disuse diminishes them; and that such modifications are inherited.
Under free nature, we can have no standard of comparison, by which to judge of the effects of long-continued
use or disuse, for we know not the parent-forms; but many animals have structures which can be explained by
the effects of disuse. As Professor Owen has remarked, there is no greater anomaly in nature than a bird that
cannot fly; yet there are several in this state. The logger-headed duck of South America can only flap along
the surface of the water, and has its wings in nearly the same condition as the domestic Aylesbury duck. As
the larger ground-feeding birds seldom take flight except to escape danger, I believe that the nearly wingless
condition of several birds, which now inhabit or have lately inhabited several oceanic islands, tenanted by no
beast of prey, has been caused by disuse. The ostrich indeed inhabits continents and is exposed to danger from
which it cannot escape by flight, but by kicking it can defend itself from enemies, as well as any of the smaller
quadrupeds. We may imagine that the early progenitor of the ostrich had habits like those of a bustard, and
that as natural selection increased in successive generations the size and weight of its body, its legs were used
more, and its wings less, until they became incapable of flight.
Kirby has remarked (and I have observed the same fact) that the anterior tarsi, or feet, of many male
dung-feeding beetles are very often broken off; he examined seventeen specimens in his own collection, and
not one had even a relic left. In the Onites apelles the tarsi are so habitually lost, that the insect has been
described as not having them. In some other genera they are present, but in a rudimentary condition. In the
Ateuchus or sacred beetle of the Egyptians, they are totally deficient. There is not sufficient evidence to
induce us to believe that mutilations are ever inherited; and I should prefer explaining the entire absence of the
anterior tarsi in Ateuchus, and their rudimentary condition in some other genera, by the long-continued effects
of disuse in their progenitors; for as the tarsi are almost always lost in many dung-feeding beetles, they must
be lost early in life, and therefore cannot be much used by these insects.
In some cases we might easily put down to disuse modifications of structure which are wholly, or mainly, due
to natural selection. Mr. Wollaston has discovered the remarkable fact that 200 beetles, out of the 550 species
inhabiting Madeira, are so far deficient in wings that they cannot fly; and that of the twenty-nine endemic
genera, no less than twenty-three genera have all their species in this condition! Several facts, namely, that
beetles in many parts of the world are very frequently blown to sea and perish; that the beetles in Madeira, as
observed by Mr. Wollaston, lie much concealed, until the wind lulls and the sun shines; that the proportion of
wingless beetles is larger on the exposed Dezertas than in Madeira itself; and especially the extraordinary fact,
so strongly insisted on by Mr. Wollaston, of the almost entire absence of certain large groups of beetles,
elsewhere excessively numerous, and which groups have habits of life almost necessitating frequent
flight;--these several considerations have made me believe that the wingless condition of so many Madeira
beetles is mainly due to the action of natural selection, but combined probably with disuse. For during
thousands of successive generations each individual beetle which flew least, either from its wings having been
ever so little less perfectly developed or from indolent habit, will have had the best chance of surviving from
not being blown out to sea; and, on the other hand, those beetles which most readily took to flight will
oftenest have been blown to sea and thus have been destroyed.
The insects in Madeira which are not ground-feeders, and which, as the flower-feeding coleoptera and
lepidoptera, must habitually use their wings to gain their subsistence, have, as Mr. Wollaston suspects, their
wings not at all reduced, but even enlarged. This is quite compatible with the action of natural selection. For
when a new insect first arrived on the island, the tendency of natural selection to enlarge or to reduce the
wings, would depend on whether a greater number of individuals were saved by successfully battling with the
winds, or by giving up the attempt and rarely or never flying. As with mariners shipwrecked near a coast, it
would have been better for the good swimmers if they had been able to swim still further, whereas it would
have been better for the bad swimmers if they had not been able to swim at all and had stuck to the wreck.
The eyes of moles and of some burrowing rodents are rudimentary in size, and in some cases are quite
covered up by skin and fur. This state of the eyes is probably due to gradual reduction from disuse, but aided
perhaps by natural selection. In South America, a burrowing rodent, the tuco-tuco, or Ctenomys, is even more
subterranean in its habits than the mole; and I was assured by a Spaniard, who had often caught them, that
they were frequently blind; one which I kept alive was certainly in this condition, the cause, as appeared on
dissection, having been inflammation of the nictitating membrane. As frequent inflammation of the eyes must
be injurious to any animal, and as eyes are certainly not indispensable to animals with subterranean habits, a
reduction in their size with the adhesion of the eyelids and growth of fur over them, might in such case be an
advantage; and if so, natural selection would constantly aid the effects of disuse.
It is well known that several animals, belonging to the most different classes, which inhabit the caves of Styria
and of Kentucky, are blind. In some of the crabs the foot-stalk for the eye remains, though the eye is gone; the
stand for the telescope is there, though the telescope with its glasses has been lost. As it is difficult to imagine
that eyes, though useless, could be in any way injurious to animals living in darkness, I attribute their loss
wholly to disuse. In one of the blind animals, namely, the cave-rat, the eyes are of immense size; and
Professor Silliman thought that it regained, after living some days in the light, some slight power of vision. In
the same manner as in Madeira the wings of some of the insects have been enlarged, and the wings of others
have been reduced by natural selection aided by use and disuse, so in the case of the cave-rat natural selection
seems to have struggled with the loss of light and to have increased the size of the eyes; whereas with all the
other inhabitants of the caves, disuse by itself seems to have done its work.
It is difficult to imagine conditions of life more similar than deep limestone caverns under a nearly similar
climate; so that on the common view of the blind animals having been separately created for the American
and European caverns, close similarity in their organisation and affinities might have been expected; but, as
Schiodte and others have remarked, this is not the case, and the cave-insects of the two continents are not
more closely allied than might have been anticipated from the general resemblance of the other inhabitants of
North America and Europe. On my view we must suppose that American animals, having ordinary powers of
vision, slowly migrated by successive generations from the outer world into the deeper and deeper recesses of
the Kentucky caves, as did European animals into the caves of Europe. We have some evidence of this
gradation of habit; for, as Schiodte remarks, "animals not far remote from ordinary forms, prepare the
transition from light to darkness. Next follow those that are constructed for twilight; and, last of all, those
destined for total darkness." By the time that an animal had reached, after numberless generations, the deepest
recesses, disuse will on this view have more or less perfectly obliterated its eyes, and natural selection will
often have effected other changes, such as an increase in the length of the antennae or palpi, as a
compensation for blindness. Notwithstanding such modifications, we might expect still to see in the
cave-animals of America, affinities to the other inhabitants of that continent, and in those of Europe, to the
inhabitants of the European continent. And this is the case with some of the American cave-animals, as I hear
from Professor Dana; and some of the European cave-insects are very closely allied to those of the
surrounding country. It would be most difficult to give any rational explanation of the affinities of the blind
cave-animals to the other inhabitants of the two continents on the ordinary view of their independent creation.
That several of the inhabitants of the caves of the Old and New Worlds should be closely related, we might
expect from the well-known relationship of most of their other productions. Far from feeling any surprise that
some of the cave-animals should be very anomalous, as Agassiz has remarked in regard to the blind fish, the
Amblyopsis, and as is the case with the blind Proteus with reference to the reptiles of Europe, I am only
surprised that more wrecks of ancient life have not been preserved, owing to the less severe competition to
which the inhabitants of these dark abodes will probably have been exposed.
ACCLIMATISATION.
Habit is hereditary with plants, as in the period of flowering, in the amount of rain requisite for seeds to
germinate, in the time of sleep, etc., and this leads me to say a few words on acclimatisation. As it is
extremely common for species of the same genus to inhabit very hot and very cold countries, and as I believe
that all the species of the same genus have descended from a single parent, if this view be correct,
acclimatisation must be readily effected during long-continued descent. It is notorious that each species is
adapted to the climate of its own home: species from an arctic or even from a temperate region cannot endure
a tropical climate, or conversely. So again, many succulent plants cannot endure a damp climate. But the
degree of adaptation of species to the climates under which they live is often overrated. We may infer this
from our frequent inability to predict whether or not an imported plant will endure our climate, and from the
number of plants and animals brought from warmer countries which here enjoy good health. We have reason
to believe that species in a state of nature are limited in their ranges by the competition of other organic beings
quite as much as, or more than, by adaptation to particular climates. But whether or not the adaptation be
generally very close, we have evidence, in the case of some few plants, of their becoming, to a certain extent,
naturally habituated to different temperatures, or becoming acclimatised: thus the pines and rhododendrons,
raised from seed collected by Dr. Hooker from trees growing at different heights on the Himalaya, were found
in this country to possess different constitutional powers of resisting cold. Mr. Thwaites informs me that he
has observed similar facts in Ceylon, and analogous observations have been made by Mr. H. C. Watson on
European species of plants brought from the Azores to England. In regard to animals, several authentic cases
could be given of species within historical times having largely extended their range from warmer to cooler
latitudes, and conversely; but we do not positively know that these animals were strictly adapted to their
native climate, but in all ordinary cases we assume such to be the case; nor do we know that they have
subsequently become acclimatised to their new homes.
As I believe that our domestic animals were originally chosen by uncivilised man because they were useful
and bred readily under confinement, and not because they were subsequently found capable of far-extended
transportation, I think the common and extraordinary capacity in our domestic animals of not only
withstanding the most different climates but of being perfectly fertile (a far severer test) under them, may be
used as an argument that a large proportion of other animals, now in a state of nature, could easily be brought
to bear widely different climates. We must not, however, push the foregoing argument too far, on account of
the probable origin of some of our domestic animals from several wild stocks: the blood, for instance, of a
tropical and arctic wolf or wild dog may perhaps be mingled in our domestic breeds. The rat and mouse
cannot be considered as domestic animals, but they have been transported by man to many parts of the world,
and now have a far wider range than any other rodent, living free under the cold climate of Faroe in the north
and of the Falklands in the south, and on many islands in the torrid zones. Hence I am inclined to look at
adaptation to any special climate as a quality readily grafted on an innate wide flexibility of constitution,
which is common to most animals. On this view, the capacity of enduring the most different climates by man
himself and by his domestic animals, and such facts as that former species of the elephant and rhinoceros were
capable of enduring a glacial climate, whereas the living species are now all tropical or sub-tropical in their
habits, ought not to be looked at as anomalies, but merely as examples of a very common flexibility of
constitution, brought, under peculiar circumstances, into play.
How much of the acclimatisation of species to any peculiar climate is due to mere habit, and how much to the
natural selection of varieties having different innate constitutions, and how much to both means combined, is
a very obscure question. That habit or custom has some influence I must believe, both from analogy, and from
the incessant advice given in agricultural works, even in the ancient Encyclopaedias of China, to be very
cautious in transposing animals from one district to another; for it is not likely that man should have
succeeded in selecting so many breeds and sub-breeds with constitutions specially fitted for their own
districts: the result must, I think, be due to habit. On the other hand, I can see no reason to doubt that natural
selection will continually tend to preserve those individuals which are born with constitutions best adapted to
their native countries. In treatises on many kinds of cultivated plants, certain varieties are said to withstand
certain climates better than others: this is very strikingly shown in works on fruit trees published in the United
States, in which certain varieties are habitually recommended for the northern, and others for the southern
States; and as most of these varieties are of recent origin, they cannot owe their constitutional differences to
habit. The case of the Jerusalem artichoke, which is never propagated by seed, and of which consequently new
varieties have not been produced, has even been advanced--for it is now as tender as ever it was--as proving
that acclimatisation cannot be effected! The case, also, of the kidney-bean has been often cited for a similar
purpose, and with much greater weight; but until some one will sow, during a score of generations, his
kidney-beans so early that a very large proportion are destroyed by frost, and then collect seed from the few
survivors, with care to prevent accidental crosses, and then again get seed from these seedlings, with the same
precautions, the experiment cannot be said to have been even tried. Nor let it be supposed that no differences
in the constitution of seedling kidney-beans ever appear, for an account has been published how much more
hardy some seedlings appeared to be than others.
On the whole, I think we may conclude that habit, use, and disuse, have, in some cases, played a considerable
part in the modification of the constitution, and of the structure of various organs; but that the effects of use
and disuse have often been largely combined with, and sometimes overmastered by, the natural selection of
innate differences.
CORRELATION OF GROWTH.
I mean by this expression that the whole organisation is so tied together during its growth and development,
that when slight variations in any one part occur, and are accumulated through natural selection, other parts
become modified. This is a very important subject, most imperfectly understood. The most obvious case is,
that modifications accumulated solely for the good of the young or larva, will, it may safely be concluded,
affect the structure of the adult; in the same manner as any malconformation affecting the early embryo,
seriously affects the whole organisation of the adult. The several parts of the body which are homologous, and
which, at an early embryonic period, are alike, seem liable to vary in an allied manner: we see this in the right
and left sides of the body varying in the same manner; in the front and hind legs, and even in the jaws and
limbs, varying together, for the lower jaw is believed to be homologous with the limbs. These tendencies, I do
not doubt, may be mastered more or less completely by natural selection: thus a family of stags once existed
with an antler only on one side; and if this had been of any great use to the breed it might probably have been
rendered permanent by natural selection.
Homologous parts, as has been remarked by some authors, tend to cohere; this is often seen in monstrous
plants; and nothing is more common than the union of homologous parts in normal structures, as the union of
the petals of the corolla into a tube. Hard parts seem to affect the form of adjoining soft parts; it is believed by
some authors that the diversity in the shape of the pelvis in birds causes the remarkable diversity in the shape
of their kidneys. Others believe that the shape of the pelvis in the human mother influences by pressure the
shape of the head of the child. In snakes, according to Schlegel, the shape of the body and the manner of
swallowing determine the position of several of the most important viscera.
The nature of the bond of correlation is very frequently quite obscure. M. Is. Geoffroy St. Hilaire has forcibly
remarked, that certain malconformations very frequently, and that others rarely coexist, without our being able
to assign any reason. What can be more singular than the relation between blue eyes and deafness in cats, and
the tortoise-shell colour with the female sex; the feathered feet and skin between the outer toes in pigeons, and
the presence of more or less down on the young birds when first hatched, with the future colour of their
plumage; or, again, the relation between the hair and teeth in the naked Turkish dog, though here probably
homology comes into play? With respect to this latter case of correlation, I think it can hardly be accidental,
that if we pick out the two orders of mammalia which are most abnormal in their dermal coverings, viz.
Cetacea (whales) and Edentata (armadilloes, scaly ant-eaters, etc.), that these are likewise the most abnormal
in their teeth.
I know of no case better adapted to show the importance of the laws of correlation in modifying important
structures, independently of utility and, therefore, of natural selection, than that of the difference between the
outer and inner flowers in some Compositous and Umbelliferous plants. Every one knows the difference in
the ray and central florets of, for instance, the daisy, and this difference is often accompanied with the
abortion of parts of the flower. But, in some Compositous plants, the seeds also differ in shape and sculpture;
and even the ovary itself, with its accessory parts, differs, as has been described by Cassini. These differences
have been attributed by some authors to pressure, and the shape of the seeds in the ray-florets in some
Compositae countenances this idea; but, in the case of the corolla of the Umbelliferae, it is by no means, as
Dr. Hooker informs me, in species with the densest heads that the inner and outer flowers most frequently
differ. It might have been thought that the development of the ray-petals by drawing nourishment from certain
other parts of the flower had caused their abortion; but in some Compositae there is a difference in the seeds
of the outer and inner florets without any difference in the corolla. Possibly, these several differences may be
connected with some difference in the flow of nutriment towards the central and external flowers: we know, at
least, that in irregular flowers, those nearest to the axis are oftenest subject to peloria, and become regular. I
may add, as an instance of this, and of a striking case of correlation, that I have recently observed in some
garden pelargoniums, that the central flower of the truss often loses the patches of darker colour in the two
upper petals; and that when this occurs, the adherent nectary is quite aborted; when the colour is absent from
only one of the two upper petals, the nectary is only much shortened.
With respect to the difference in the corolla of the central and exterior flowers of a head or umbel, I do not
feel at all sure that C. C. Sprengel's idea that the ray-florets serve to attract insects, whose agency is highly
advantageous in the fertilisation of plants of these two orders, is so far-fetched, as it may at first appear: and if
it be advantageous, natural selection may have come into play. But in regard to the differences both in the
internal and external structure of the seeds, which are not always correlated with any differences in the
flowers, it seems impossible that they can be in any way advantageous to the plant: yet in the Umbelliferae
these differences are of such apparent importance--the seeds being in some cases, according to Tausch,
orthospermous in the exterior flowers and coelospermous in the central flowers,--that the elder De Candolle
founded his main divisions of the order on analogous differences. Hence we see that modifications of
structure, viewed by systematists as of high value, may be wholly due to unknown laws of correlated growth,
and without being, as far as we can see, of the slightest service to the species.
We may often falsely attribute to correlation of growth, structures which are common to whole groups of
species, and which in truth are simply due to inheritance; for an ancient progenitor may have acquired through
natural selection some one modification in structure, and, after thousands of generations, some other and
independent modification; and these two modifications, having been transmitted to a whole group of
descendants with diverse habits, would naturally be thought to be correlated in some necessary manner. So,
again, I do not doubt that some apparent correlations, occurring throughout whole orders, are entirely due to
the manner alone in which natural selection can act. For instance, Alph. De Candolle has remarked that
winged seeds are never found in fruits which do not open: I should explain the rule by the fact that seeds
could not gradually become winged through natural selection, except in fruits which opened; so that the
individual plants producing seeds which were a little better fitted to be wafted further, might get an advantage
over those producing seed less fitted for dispersal; and this process could not possibly go on in fruit which did
not open.
The elder Geoffroy and Goethe propounded, at about the same period, their law of compensation or
balancement of growth; or, as Goethe expressed it, "in order to spend on one side, nature is forced to
economise on the other side." I think this holds true to a certain extent with our domestic productions: if
nourishment flows to one part or organ in excess, it rarely flows, at least in excess, to another part; thus it is
difficult to get a cow to give much milk and to fatten readily. The same varieties of the cabbage do not yield
abundant and nutritious foliage and a copious supply of oil-bearing seeds. When the seeds in our fruits
become atrophied, the fruit itself gains largely in size and quality. In our poultry, a large tuft of feathers on the
head is generally accompanied by a diminished comb, and a large beard by diminished wattles. With species
in a state of nature it can hardly be maintained that the law is of universal application; but many good
observers, more especially botanists, believe in its truth. I will not, however, here give any instances, for I see
hardly any way of distinguishing between the effects, on the one hand, of a part being largely developed
through natural selection and another and adjoining part being reduced by this same process or by disuse, and,
on the other hand, the actual withdrawal of nutriment from one part owing to the excess of growth in another
and adjoining part.
I suspect, also, that some of the cases of compensation which have been advanced, and likewise some other
facts, may be merged under a more general principle, namely, that natural selection is continually trying to
economise in every part of the organisation. If under changed conditions of life a structure before useful
becomes less useful, any diminution, however slight, in its development, will be seized on by natural
selection, for it will profit the individual not to have its nutriment wasted in building up an useless structure. I
can thus only understand a fact with which I was much struck when examining cirripedes, and of which many
other instances could be given: namely, that when a cirripede is parasitic within another and is thus protected,
it loses more or less completely its own shell or carapace. This is the case with the male Ibla, and in a truly
extraordinary manner with the Proteolepas: for the carapace in all other cirripedes consists of the three
highly-important anterior segments of the head enormously developed, and furnished with great nerves and
muscles; but in the parasitic and protected Proteolepas, the whole anterior part of the head is reduced to the
merest rudiment attached to the bases of the prehensile antennae. Now the saving of a large and complex
structure, when rendered superfluous by the parasitic habits of the Proteolepas, though effected by slow steps,
would be a decided advantage to each successive individual of the species; for in the struggle for life to which
every animal is exposed, each individual Proteolepas would have a better chance of supporting itself, by less
nutriment being wasted in developing a structure now become useless.
Thus, as I believe, natural selection will always succeed in the long run in reducing and saving every part of
the organisation, as soon as it is rendered superfluous, without by any means causing some other part to be
largely developed in a corresponding degree. And, conversely, that natural selection may perfectly well
succeed in largely developing any organ, without requiring as a necessary compensation the reduction of some
adjoining part.
It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both in varieties and in species, that when any
part or organ is repeated many times in the structure of the same individual (as the vertebrae in snakes, and the
stamens in polyandrous flowers) the number is variable; whereas the number of the same part or organ, when
it occurs in lesser numbers, is constant. The same author and some botanists have further remarked that
multiple parts are also very liable to variation in structure. Inasmuch as this "vegetative repetition," to use
Professor Owen's expression, seems to be a sign of low organisation; the foregoing remark seems connected
with the very general opinion of naturalists, that beings low in the scale of nature are more variable than those
which are higher. I presume that lowness in this case means that the several parts of the organisation have
been but little specialised for particular functions; and as long as the same part has to perform diversified
work, we can perhaps see why it should remain variable, that is, why natural selection should have preserved
or rejected each little deviation of form less carefully than when the part has to serve for one special purpose
alone. In the same way that a knife which has to cut all sorts of things may be of almost any shape; whilst a
tool for some particular object had better be of some particular shape. Natural selection, it should never be
forgotten, can act on each part of each being, solely through and for its advantage.
Rudimentary parts, it has been stated by some authors, and I believe with truth, are apt to be highly variable.
We shall have to recur to the general subject of rudimentary and aborted organs; and I will here only add that
their variability seems to be owing to their uselessness, and therefore to natural selection having no power to
check deviations in their structure. Thus rudimentary parts are left to the free play of the various laws of
growth, to the effects of long-continued disuse, and to the tendency to reversion.
A PART DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY DEGREE OR MANNER, IN
COMPARISON WITH THE SAME PART IN ALLIED SPECIES, TENDS TO BE HIGHLY VARIABLE.
Several years ago I was much struck with a remark, nearly to the above effect, published by Mr. Waterhouse.
I infer also from an observation made by Professor Owen, with respect to the length of the arms of the
ourang-outang, that he has come to a nearly similar conclusion. It is hopeless to attempt to convince any one
of the truth of this proposition without giving the long array of facts which I have collected, and which cannot
possibly be here introduced. I can only state my conviction that it is a rule of high generality. I am aware of
several causes of error, but I hope that I have made due allowance for them. It should be understood that the
rule by no means applies to any part, however unusually developed, unless it be unusually developed in
comparison with the same part in closely allied species. Thus, the bat's wing is a most abnormal structure in
the class mammalia; but the rule would not here apply, because there is a whole group of bats having wings; it
would apply only if some one species of bat had its wings developed in some remarkable manner in
comparison with the other species of the same genus. The rule applies very strongly in the case of secondary
sexual characters, when displayed in any unusual manner. The term, secondary sexual characters, used by
Hunter, applies to characters which are attached to one sex, but are not directly connected with the act of
reproduction. The rule applies to males and females; but as females more rarely offer remarkable secondary
sexual characters, it applies more rarely to them. The rule being so plainly applicable in the case of secondary
sexual characters, may be due to the great variability of these characters, whether or not displayed in any
unusual manner--of which fact I think there can be little doubt. But that our rule is not confined to secondary
sexual characters is clearly shown in the case of hermaphrodite cirripedes; and I may here add, that I
particularly attended to Mr. Waterhouse's remark, whilst investigating this Order, and I am fully convinced
that the rule almost invariably holds good with cirripedes. I shall, in my future work, give a list of the more
remarkable cases; I will here only briefly give one, as it illustrates the rule in its largest application. The
opercular valves of sessile cirripedes (rock barnacles) are, in every sense of the word, very important
structures, and they differ extremely little even in different genera; but in the several species of one genus,
Pyrgoma, these valves present a marvellous amount of diversification: the homologous valves in the different
species being sometimes wholly unlike in shape; and the amount of variation in the individuals of several of
the species is so great, that it is no exaggeration to state that the varieties differ more from each other in the
characters of these important valves than do other species of distinct genera.
As birds within the same country vary in a remarkably small degree, I have particularly attended to them, and
the rule seems to me certainly to hold good in this class. I cannot make out that it applies to plants, and this
would seriously have shaken my belief in its truth, had not the great variability in plants made it particularly
difficult to compare their relative degrees of variability.
When we see any part or organ developed in a remarkable degree or manner in any species, the fair
presumption is that it is of high importance to that species; nevertheless the part in this case is eminently liable
to variation. Why should this be so? On the view that each species has been independently created, with all its
parts as we now see them, I can see no explanation. But on the view that groups of species have descended
from other species, and have been modified through natural selection, I think we can obtain some light. In our
domestic animals, if any part, or the whole animal, be neglected and no selection be applied, that part (for
instance, the comb in the Dorking fowl) or the whole breed will cease to have a nearly uniform character. The
breed will then be said to have degenerated. In rudimentary organs, and in those which have been but little
specialised for any particular purpose, and perhaps in polymorphic groups, we see a nearly parallel natural
case; for in such cases natural selection either has not or cannot come into full play, and thus the organisation
is left in a fluctuating condition. But what here more especially concerns us is, that in our domestic animals
those points, which at the present time are undergoing rapid change by continued selection, are also eminently
liable to variation. Look at the breeds of the pigeon; see what a prodigious amount of difference there is in the
beak of the different tumblers, in the beak and wattle of the different carriers, in the carriage and tail of our
fantails, etc., these being the points now mainly attended to by English fanciers. Even in the sub-breeds, as in
the short-faced tumbler, it is notoriously difficult to breed them nearly to perfection, and frequently
individuals are born which depart widely from the standard. There may be truly said to be a constant struggle
going on between, on the one hand, the tendency to reversion to a less modified state, as well as an innate
tendency to further variability of all kinds, and, on the other hand, the power of steady selection to keep the
breed true. In the long run selection gains the day, and we do not expect to fail so far as to breed a bird as
coarse as a common tumbler from a good short-faced strain. But as long as selection is rapidly going on, there
may always be expected to be much variability in the structure undergoing modification. It further deserves
notice that these variable characters, produced by man's selection, sometimes become attached, from causes
quite unknown to us, more to one sex than to the other, generally to the male sex, as with the wattle of carriers
and the enlarged crop of pouters.
Now let us turn to nature. When a part has been developed in an extraordinary manner in any one species,
compared with the other species of the same genus, we may conclude that this part has undergone an
extraordinary amount of modification, since the period when the species branched off from the common
progenitor of the genus. This period will seldom be remote in any extreme degree, as species very rarely
endure for more than one geological period. An extraordinary amount of modification implies an unusually
large and long-continued amount of variability, which has continually been accumulated by natural selection
for the benefit of the species. But as the variability of the extraordinarily-developed part or organ has been so
great and long-continued within a period not excessively remote, we might, as a general rule, expect still to
find more variability in such parts than in other parts of the organisation, which have remained for a much
longer period nearly constant. And this, I am convinced, is the case. That the struggle between natural
selection on the one hand, and the tendency to reversion and variability on the other hand, will in the course of
time cease; and that the most abnormally developed organs may be made constant, I can see no reason to
doubt. Hence when an organ, however abnormal it may be, has been transmitted in approximately the same
condition to many modified descendants, as in the case of the wing of the bat, it must have existed, according
to my theory, for an immense period in nearly the same state; and thus it comes to be no more variable than
any other structure. It is only in those cases in which the modification has been comparatively recent and
extraordinarily great that we ought to find the GENERATIVE VARIABILITY, as it may be called, still
present in a high degree. For in this case the variability will seldom as yet have been fixed by the continued
selection of the individuals varying in the required manner and degree, and by the continued rejection of those
tending to revert to a former and less modified condition.
The principle included in these remarks may be extended. It is notorious that specific characters are more
variable than generic. To explain by a simple example what is meant. If some species in a large genus of
plants had blue flowers and some had red, the colour would be only a specific character, and no one would be
surprised at one of the blue species varying into red, or conversely; but if all the species had blue flowers, the
colour would become a generic character, and its variation would be a more unusual circumstance. I have
chosen this example because an explanation is not in this case applicable, which most naturalists would
advance, namely, that specific characters are more variable than generic, because they are taken from parts of
less physiological importance than those commonly used for classing genera. I believe this explanation is
partly, yet only indirectly, true; I shall, however, have to return to this subject in our chapter on Classification.
It would be almost superfluous to adduce evidence in support of the above statement, that specific characters
are more variable than generic; but I have repeatedly noticed in works on natural history, that when an author
has remarked with surprise that some IMPORTANT organ or part, which is generally very constant
throughout large groups of species, has DIFFERED considerably in closely-allied species, that it has, also,
been VARIABLE in the individuals of some of the species. And this fact shows that a character, which is
generally of generic value, when it sinks in value and becomes only of specific value, often becomes variable,
though its physiological importance may remain the same. Something of the same kind applies to
monstrosities: at least Is. Geoffroy St. Hilaire seems to entertain no doubt, that the more an organ normally
differs in the different species of the same group, the more subject it is to individual anomalies.
On the ordinary view of each species having been independently created, why should that part of the structure,
which differs from the same part in other independently-created species of the same genus, be more variable
than those parts which are closely alike in the several species? I do not see that any explanation can be given.
But on the view of species being only strongly marked and fixed varieties, we might surely expect to find
them still often continuing to vary in those parts of their structure which have varied within a moderately
recent period, and which have thus come to differ. Or to state the case in another manner:--the points in which
all the species of a genus resemble each other, and in which they differ from the species of some other genus,
are called generic characters; and these characters in common I attribute to inheritance from a common
progenitor, for it can rarely have happened that natural selection will have modified several species, fitted to
more or less widely-different habits, in exactly the same manner: and as these so-called generic characters
have been inherited from a remote period, since that period when the species first branched off from their
common progenitor, and subsequently have not varied or come to differ in any degree, or only in a slight
degree, it is not probable that they should vary at the present day. On the other hand, the points in which
species differ from other species of the same genus, are called specific characters; and as these specific
characters have varied and come to differ within the period of the branching off of the species from a common
progenitor, it is probable that they should still often be in some degree variable,--at least more variable than
those parts of the organisation which have for a very long period remained constant.
In connexion with the present subject, I will make only two other remarks. I think it will be admitted, without
my entering on details, that secondary sexual characters are very variable; I think it also will be admitted that
species of the same group differ from each other more widely in their secondary sexual characters, than in
other parts of their organisation; compare, for instance, the amount of difference between the males of
gallinaceous birds, in which secondary sexual characters are strongly displayed, with the amount of difference
between their females; and the truth of this proposition will be granted. The cause of the original variability of
secondary sexual characters is not manifest; but we can see why these characters should not have been
rendered as constant and uniform as other parts of the organisation; for secondary sexual characters have been
accumulated by sexual selection, which is less rigid in its action than ordinary selection, as it does not entail
death, but only gives fewer offspring to the less favoured males. Whatever the cause may be of the variability
of secondary sexual characters, as they are highly variable, sexual selection will have had a wide scope for
action, and may thus readily have succeeded in giving to the species of the same group a greater amount of
difference in their sexual characters, than in other parts of their structure.
It is a remarkable fact, that the secondary sexual differences between the two sexes of the same species are
generally displayed in the very same parts of the organisation in which the different species of the same genus
differ from each other. Of this fact I will give in illustration two instances, the first which happen to stand on
my list; and as the differences in these cases are of a very unusual nature, the relation can hardly be accidental.
The same number of joints in the tarsi is a character generally common to very large groups of beetles, but in
the Engidae, as Westwood has remarked, the number varies greatly; and the number likewise differs in the
two sexes of the same species: again in fossorial hymenoptera, the manner of neuration of the wings is a
character of the highest importance, because common to large groups; but in certain genera the neuration
differs in the different species, and likewise in the two sexes of the same species. This relation has a clear
meaning on my view of the subject: I look at all the species of the same genus as having as certainly
descended from the same progenitor, as have the two sexes of any one of the species. Consequently, whatever
part of the structure of the common progenitor, or of its early descendants, became variable; variations of this
part would it is highly probable, be taken advantage of by natural and sexual selection, in order to fit the
several species to their several places in the economy of nature, and likewise to fit the two sexes of the same
species to each other, or to fit the males and females to different habits of life, or the males to struggle with
other males for the possession of the females.
Finally, then, I conclude that the greater variability of specific characters, or those which distinguish species
from species, than of generic characters, or those which the species possess in common;--that the frequent
extreme variability of any part which is developed in a species in an extraordinary manner in comparison with
the same part in its congeners; and the not great degree of variability in a part, however extraordinarily it may
be developed, if it be common to a whole group of species;--that the great variability of secondary sexual
characters, and the great amount of difference in these same characters between closely allied species;--that
secondary sexual and ordinary specific differences are generally displayed in the same parts of the
organisation,--are all principles closely connected together. All being mainly due to the species of the same
group having descended from a common progenitor, from whom they have inherited much in common,--to
parts which have recently and largely varied being more likely still to go on varying than parts which have
long been inherited and have not varied,--to natural selection having more or less completely, according to the
lapse of time, overmastered the tendency to reversion and to further variability,--to sexual selection being less
rigid than ordinary selection,--and to variations in the same parts having been accumulated by natural and
sexual selection, and thus adapted for secondary sexual, and for ordinary specific purposes.
DISTINCT SPECIES PRESENT ANALOGOUS VARIATIONS; AND A VARIETY OF ONE SPECIES
OFTEN ASSUMES SOME OF THE CHARACTERS OF AN ALLIED SPECIES, OR REVERTS TO SOME
OF THE CHARACTERS OF AN EARLY PROGENITOR.
These propositions will be most readily understood by looking to our domestic races. The most distinct breeds
of pigeons, in countries most widely apart, present sub-varieties with reversed feathers on the head and
feathers on the feet,--characters not possessed by the aboriginal rock-pigeon; these then are analogous
variations in two or more distinct races. The frequent presence of fourteen or even sixteen tail-feathers in the
pouter, may be considered as a variation representing the normal structure of another race, the fantail. I
presume that no one will doubt that all such analogous variations are due to the several races of the pigeon
having inherited from a common parent the same constitution and tendency to variation, when acted on by
similar unknown influences. In the vegetable kingdom we have a case of analogous variation, in the enlarged
stems, or roots as commonly called, of the Swedish turnip and Ruta baga, plants which several botanists rank
as varieties produced by cultivation from a common parent: if this be not so, the case will then be one of
analogous variation in two so-called distinct species; and to these a third may be added, namely, the common
turnip. According to the ordinary view of each species having been independently created, we should have to
attribute this similarity in the enlarged stems of these three plants, not to the vera causa of community of
descent, and a consequent tendency to vary in a like manner, but to three separate yet closely related acts of
creation.
With pigeons, however, we have another case, namely, the occasional appearance in all the breeds, of
slaty-blue birds with two black bars on the wings, a white rump, a bar at the end of the tail, with the outer
feathers externally edged near their bases with white. As all these marks are characteristic of the parent
rock-pigeon, I presume that no one will doubt that this is a case of reversion, and not of a new yet analogous
variation appearing in the several breeds. We may I think confidently come to this conclusion, because, as we
have seen, these coloured marks are eminently liable to appear in the crossed offspring of two distinct and
differently coloured breeds; and in this case there is nothing in the external conditions of life to cause the
reappearance of the slaty-blue, with the several marks, beyond the influence of the mere act of crossing on the
laws of inheritance.
No doubt it is a very surprising fact that characters should reappear after having been lost for many, perhaps
for hundreds of generations. But when a breed has been crossed only once by some other breed, the offspring
occasionally show a tendency to revert in character to the foreign breed for many generations--some say, for a
dozen or even a score of generations. After twelve generations, the proportion of blood, to use a common
expression, of any one ancestor, is only 1 in 2048; and yet, as we see, it is generally believed that a tendency
to reversion is retained by this very small proportion of foreign blood. In a breed which has not been crossed,
but in which BOTH parents have lost some character which their progenitor possessed, the tendency, whether
strong or weak, to reproduce the lost character might be, as was formerly remarked, for all that we can see to
the contrary, transmitted for almost any number of generations. When a character which has been lost in a
breed, reappears after a great number of generations, the most probable hypothesis is, not that the offspring
suddenly takes after an ancestor some hundred generations distant, but that in each successive generation there
has been a tendency to reproduce the character in question, which at last, under unknown favourable
conditions, gains an ascendancy. For instance, it is probable that in each generation of the barb-pigeon, which
produces most rarely a blue and black-barred bird, there has been a tendency in each generation in the
plumage to assume this colour. This view is hypothetical, but could be supported by some facts; and I can see
no more abstract improbability in a tendency to produce any character being inherited for an endless number
of generations, than in quite useless or rudimentary organs being, as we all know them to be, thus inherited.
Indeed, we may sometimes observe a mere tendency to produce a rudiment inherited: for instance, in the
common snapdragon (Antirrhinum) a rudiment of a fifth stamen so often appears, that this plant must have an
inherited tendency to produce it.
As all the species of the same genus are supposed, on my theory, to have descended from a common parent, it
might be expected that they would occasionally vary in an analogous manner; so that a variety of one species
would resemble in some of its characters another species; this other species being on my view only a
well-marked and permanent variety. But characters thus gained would probably be of an unimportant nature,
for the presence of all important characters will be governed by natural selection, in accordance with the
diverse habits of the species, and will not be left to the mutual action of the conditions of life and of a similar
inherited constitution. It might further be expected that the species of the same genus would occasionally
exhibit reversions to lost ancestral characters. As, however, we never know the exact character of the common
ancestor of a group, we could not distinguish these two cases: if, for instance, we did not know that the
rock-pigeon was not feather-footed or turn-crowned, we could not have told, whether these characters in our
domestic breeds were reversions or only analogous variations; but we might have inferred that the blueness
was a case of reversion, from the number of the markings, which are correlated with the blue tint, and which it
does not appear probable would all appear together from simple variation. More especially we might have
inferred this, from the blue colour and marks so often appearing when distinct breeds of diverse colours are
crossed. Hence, though under nature it must generally be left doubtful, what cases are reversions to an
anciently existing character, and what are new but analogous variations, yet we ought, on my theory,
sometimes to find the varying offspring of a species assuming characters (either from reversion or from
analogous variation) which already occur in some other members of the same group. And this undoubtedly is
the case in nature.
A considerable part of the difficulty in recognising a variable species in our systematic works, is due to its
varieties mocking, as it were, some of the other species of the same genus. A considerable catalogue, also,
could be given of forms intermediate between two other forms, which themselves must be doubtfully ranked
as either varieties or species; and this shows, unless all these forms be considered as independently created
species, that the one in varying has assumed some of the characters of the other, so as to produce the
intermediate form. But the best evidence is afforded by parts or organs of an important and uniform nature
occasionally varying so as to acquire, in some degree, the character of the same part or organ in an allied
species. I have collected a long list of such cases; but here, as before, I lie under a great disadvantage in not
being able to give them. I can only repeat that such cases certainly do occur, and seem to me very remarkable.
I will, however, give one curious and complex case, not indeed as affecting any important character, but from
occurring in several species of the same genus, partly under domestication and partly under nature. It is a case
apparently of reversion. The ass not rarely has very distinct transverse bars on its legs, like those on the legs of
a zebra: it has been asserted that these are plainest in the foal, and from inquiries which I have made, I believe
this to be true. It has also been asserted that the stripe on each shoulder is sometimes double. The shoulder
stripe is certainly very variable in length and outline. A white ass, but NOT an albino, has been described
without either spinal or shoulder-stripe; and these stripes are sometimes very obscure, or actually quite lost, in
dark-coloured asses. The koulan of Pallas is said to have been seen with a double shoulder-stripe. The
hemionus has no shoulder-stripe; but traces of it, as stated by Mr. Blyth and others, occasionally appear: and I
have been informed by Colonel Poole that the foals of this species are generally striped on the legs, and faintly
on the shoulder. The quagga, though so plainly barred like a zebra over the body, is without bars on the legs;
but Dr. Gray has figured one specimen with very distinct zebra-like bars on the hocks.
With respect to the horse, I have collected cases in England of the spinal stripe in horses of the most distinct
breeds, and of ALL colours; transverse bars on the legs are not rare in duns, mouse-duns, and in one instance
in a chestnut: a faint shoulder-stripe may sometimes be seen in duns, and I have seen a trace in a bay horse.
My son made a careful examination and sketch for me of a dun Belgian cart-horse with a double stripe on
each shoulder and with leg-stripes; and a man, whom I can implicitly trust, has examined for me a small dun
Welch pony with THREE short parallel stripes on each shoulder.
In the north-west part of India the Kattywar breed of horses is so generally striped, that, as I hear from
Colonel Poole, who examined the breed for the Indian Government, a horse without stripes is not considered
as purely-bred. The spine is always striped; the legs are generally barred; and the shoulder-stripe, which is
sometimes double and sometimes treble, is common; the side of the face, moreover, is sometimes striped. The
stripes are plainest in the foal; and sometimes quite disappear in old horses. Colonel Poole has seen both gray
and bay Kattywar horses striped when first foaled. I have, also, reason to suspect, from information given me
by Mr. W. W. Edwards, that with the English race-horse the spinal stripe is much commoner in the foal than
in the full-grown animal. Without here entering on further details, I may state that I have collected cases of leg
and shoulder stripes in horses of very different breeds, in various countries from Britain to Eastern China; and
from Norway in the north to the Malay Archipelago in the south. In all parts of the world these stripes occur
far oftenest in duns and mouse-duns; by the term dun a large range of colour is included, from one between
brown and black to a close approach to cream-colour.
I am aware that Colonel Hamilton Smith, who has written on this subject, believes that the several breeds of
the horse have descended from several aboriginal species--one of which, the dun, was striped; and that the
above-described appearances are all due to ancient crosses with the dun stock. But I am not at all satisfied
with this theory, and should be loth to apply it to breeds so distinct as the heavy Belgian cart-horse, Welch
ponies, cobs, the lanky Kattywar race, etc., inhabiting the most distant parts of the world.
Now let us turn to the effects of crossing the several species of the horse-genus. Rollin asserts, that the
common mule from the ass and horse is particularly apt to have bars on its legs. I once saw a mule with its
legs so much striped that any one at first would have thought that it must have been the product of a zebra;
and Mr. W. C. Martin, in his excellent treatise on the horse, has given a figure of a similar mule. In four
coloured drawings, which I have seen, of hybrids between the ass and zebra, the legs were much more plainly
barred than the rest of the body; and in one of them there was a double shoulder-stripe. In Lord Moreton's
famous hybrid from a chestnut mare and male quagga, the hybrid, and even the pure offspring subsequently
produced from the mare by a black Arabian sire, were much more plainly barred across the legs than is even
the pure quagga. Lastly, and this is another most remarkable case, a hybrid has been figured by Dr. Gray (and
he informs me that he knows of a second case) from the ass and the hemionus; and this hybrid, though the ass
seldom has stripes on its legs and the hemionus has none and has not even a shoulder-stripe, nevertheless had
all four legs barred, and had three short shoulder-stripes, like those on the dun Welch pony, and even had
some zebra-like stripes on the sides of its face. With respect to this last fact, I was so convinced that not even
a stripe of colour appears from what would commonly be called an accident, that I was led solely from the
occurrence of the face-stripes on this hybrid from the ass and hemionus, to ask Colonel Poole whether such
face-stripes ever occur in the eminently striped Kattywar breed of horses, and was, as we have seen, answered
in the affirmative.
What now are we to say to these several facts? We see several very distinct species of the horse-genus
becoming, by simple variation, striped on the legs like a zebra, or striped on the shoulders like an ass. In the
horse we see this tendency strong whenever a dun tint appears--a tint which approaches to that of the general
colouring of the other species of the genus. The appearance of the stripes is not accompanied by any change of
form or by any other new character. We see this tendency to become striped most strongly displayed in
hybrids from between several of the most distinct species. Now observe the case of the several breeds of
pigeons: they are descended from a pigeon (including two or three sub-species or geographical races) of a
bluish colour, with certain bars and other marks; and when any breed assumes by simple variation a bluish
tint, these bars and other marks invariably reappear; but without any other change of form or character. When
the oldest and truest breeds of various colours are crossed, we see a strong tendency for the blue tint and bars
and marks to reappear in the mongrels. I have stated that the most probable hypothesis to account for the
reappearance of very ancient characters, is--that there is a TENDENCY in the young of each successive
generation to produce the long-lost character, and that this tendency, from unknown causes, sometimes
prevails. And we have just seen that in several species of the horse-genus the stripes are either plainer or
appear more commonly in the young than in the old. Call the breeds of pigeons, some of which have bred true
for centuries, species; and how exactly parallel is the case with that of the species of the horse-genus! For
myself, I venture confidently to look back thousands on thousands of generations, and I see an animal striped
like a zebra, but perhaps otherwise very differently constructed, the common parent of our domestic horse,
whether or not it be descended from one or more wild stocks, of the ass, the hemionus, quagga, and zebra.
He who believes that each equine species was independently created, will, I presume, assert that each species
has been created with a tendency to vary, both under nature and under domestication, in this particular
manner, so as often to become striped like other species of the genus; and that each has been created with a
strong tendency, when crossed with species inhabiting distant quarters of the world, to produce hybrids
resembling in their stripes, not their own parents, but other species of the genus. To admit this view is, as it
seems to me, to reject a real for an unreal, or at least for an unknown, cause. It makes the works of God a mere
mockery and deception; I would almost as soon believe with the old and ignorant cosmogonists, that fossil
shells had never lived, but had been created in stone so as to mock the shells now living on the sea-shore.