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.
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.
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