CHAPTER 2.
VARIATION UNDER NATURE.
Variability. Individual differences. Doubtful species. Wide ranging, much diffused, and common species vary
most. Species of the larger genera in any country vary more than the species of the smaller genera. Many of
the species of the larger genera resemble varieties in being very closely, but unequally, related to each other,
and in having restricted ranges.
Before applying the principles arrived at in the last chapter to organic beings in a state of nature, we must
briefly discuss whether these latter are subject to any variation. To treat this subject at all properly, a long
catalogue of dry facts should be given; but these I shall reserve for my future work. Nor shall I here discuss
the various definitions which have been given of the term species. No one definition has as yet satisfied all
naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. Generally the term
includes the unknown element of a distinct act of creation. The term "variety" is almost equally difficult to
define; but here community of descent is almost universally implied, though it can rarely be proved. We have
also what are called monstrosities; but they graduate into varieties. By a monstrosity I presume is meant some
considerable deviation of structure in one part, either injurious to or not useful to the species, and not
generally propagated. Some authors use the term "variation" in a technical sense, as implying a modification
directly due to the physical conditions of life; and "variations" in this sense are supposed not to be inherited:
but who can say that the dwarfed condition of shells in the brackish waters of the Baltic, or dwarfed plants on
Alpine summits, or the thicker fur of an animal from far northwards, would not in some cases be inherited for
at least some few generations? and in this case I presume that the form would be called a variety.
Again, we have many slight differences which may be called individual differences, such as are known
frequently to appear in the offspring from the same parents, or which may be presumed to have thus arisen,
from being frequently observed in the individuals of the same species inhabiting the same confined locality.
No one supposes that all the individuals of the same species are cast in the very same mould. These individual
differences are highly important for us, as they afford materials for natural selection to accumulate, in the
same manner as man can accumulate in any given direction individual differences in his domesticated
productions. These individual differences generally affect what naturalists consider unimportant parts; but I
could show by a long catalogue of facts, that parts which must be called important, whether viewed under a
physiological or classificatory point of view, sometimes vary in the individuals of the same species. I am
convinced that the most experienced naturalist would be surprised at the number of the cases of variability,
even in important parts of structure, which he could collect on good authority, as I have collected, during a
course of years. It should be remembered that systematists are far from pleased at finding variability in
important characters, and that there are not many men who will laboriously examine internal and important
organs, and compare them in many specimens of the same species. I should never have expected that the
branching of the main nerves close to the great central ganglion of an insect would have been variable in the
same species; I should have expected that changes of this nature could have been effected only by slow
degrees: yet quite recently Mr. Lubbock has shown a degree of variability in these main nerves in Coccus,
which may almost be compared to the irregular branching of the stem of a tree. This philosophical naturalist, I
may add, has also quite recently shown that the muscles in the larvae of certain insects are very far from
uniform. Authors sometimes argue in a circle when they state that important organs never vary; for these same
authors practically rank that character as important (as some few naturalists have honestly confessed) which
does not vary; and, under this point of view, no instance of an important part varying will ever be found: but
under any other point of view many instances assuredly can be given.
There is one point connected with individual differences, which seems to me extremely perplexing: I refer to
those genera which have sometimes been called "protean" or "polymorphic," in which the species present an
inordinate amount of variation; and hardly two naturalists can agree which forms to rank as species and which
as varieties. We may instance Rubus, Rosa, and Hieracium amongst plants, several genera of insects, and
CHAPTER 2.
VARIATION UNDER NATURE.
Variability. Individual differences. Doubtful species. Wide ranging, much diffused, and common species vary
most. Species of the larger genera in any country vary more than the species of the smaller genera. Many of
the species of the larger genera resemble varieties in being very closely, but unequally, related to each other,
and in having restricted ranges.
Before applying the principles arrived at in the last chapter to organic beings in a state of nature, we must
briefly discuss whether these latter are subject to any variation. To treat this subject at all properly, a long
catalogue of dry facts should be given; but these I shall reserve for my future work. Nor shall I here discuss
the various definitions which have been given of the term species. No one definition has as yet satisfied all
naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. Generally the term
includes the unknown element of a distinct act of creation. The term "variety" is almost equally difficult to
define; but here community of descent is almost universally implied, though it can rarely be proved. We have
also what are called monstrosities; but they graduate into varieties. By a monstrosity I presume is meant some
considerable deviation of structure in one part, either injurious to or not useful to the species, and not
generally propagated. Some authors use the term "variation" in a technical sense, as implying a modification
directly due to the physical conditions of life; and "variations" in this sense are supposed not to be inherited:
but who can say that the dwarfed condition of shells in the brackish waters of the Baltic, or dwarfed plants on
Alpine summits, or the thicker fur of an animal from far northwards, would not in some cases be inherited for
at least some few generations? and in this case I presume that the form would be called a variety.
Again, we have many slight differences which may be called individual differences, such as are known
frequently to appear in the offspring from the same parents, or which may be presumed to have thus arisen,
from being frequently observed in the individuals of the same species inhabiting the same confined locality.
No one supposes that all the individuals of the same species are cast in the very same mould. These individual
differences are highly important for us, as they afford materials for natural selection to accumulate, in the
same manner as man can accumulate in any given direction individual differences in his domesticated
productions. These individual differences generally affect what naturalists consider unimportant parts; but I
could show by a long catalogue of facts, that parts which must be called important, whether viewed under a
physiological or classificatory point of view, sometimes vary in the individuals of the same species. I am
convinced that the most experienced naturalist would be surprised at the number of the cases of variability,
even in important parts of structure, which he could collect on good authority, as I have collected, during a
course of years. It should be remembered that systematists are far from pleased at finding variability in
important characters, and that there are not many men who will laboriously examine internal and important
organs, and compare them in many specimens of the same species. I should never have expected that the
branching of the main nerves close to the great central ganglion of an insect would have been variable in the
same species; I should have expected that changes of this nature could have been effected only by slow
degrees: yet quite recently Mr. Lubbock has shown a degree of variability in these main nerves in Coccus,
which may almost be compared to the irregular branching of the stem of a tree. This philosophical naturalist, I
may add, has also quite recently shown that the muscles in the larvae of certain insects are very far from
uniform. Authors sometimes argue in a circle when they state that important organs never vary; for these same
authors practically rank that character as important (as some few naturalists have honestly confessed) which
does not vary; and, under this point of view, no instance of an important part varying will ever be found: but
under any other point of view many instances assuredly can be given.
There is one point connected with individual differences, which seems to me extremely perplexing: I refer to
those genera which have sometimes been called "protean" or "polymorphic," in which the species present an
inordinate amount of variation; and hardly two naturalists can agree which forms to rank as species and which
as varieties. We may instance Rubus, Rosa, and Hieracium amongst plants, several genera of insects, and
be sufficient? It must be admitted that many forms, considered by highly-competent judges as varieties, have
so perfectly the character of species that they are ranked by other highly-competent judges as good and true
species. But to discuss whether they are rightly called species or varieties, before any definition of these terms
has been generally accepted, is vainly to beat the air.
Many of the cases of strongly-marked varieties or doubtful species well deserve consideration; for several
interesting lines of argument, from geographical distribution, analogical variation, hybridism, etc., have been
brought to bear on the attempt to determine their rank. I will here give only a single instance,--the well-known
one of the primrose and cowslip, or Primula veris and elatior. These plants differ considerably in appearance;
they have a different flavour and emit a different odour; they flower at slightly different periods; they grow in
somewhat different stations; they ascend mountains to different heights; they have different geographical
ranges; and lastly, according to very numerous experiments made during several years by that most careful
observer Gartner, they can be crossed only with much difficulty. We could hardly wish for better evidence of
the two forms being specifically distinct. On the other hand, they are united by many intermediate links, and it
is very doubtful whether these links are hybrids; and there is, as it seems to me, an overwhelming amount of
experimental evidence, showing that they descend from common parents, and consequently must be ranked as
varieties.
Close investigation, in most cases, will bring naturalists to an agreement how to rank doubtful forms. Yet it
must be confessed, that it is in the best-known countries that we find the greatest number of forms of doubtful
value. I have been struck with the fact, that if any animal or plant in a state of nature be highly useful to man,
or from any cause closely attract his attention, varieties of it will almost universally be found recorded. These
varieties, moreover, will be often ranked by some authors as species. Look at the common oak, how closely it
has been studied; yet a German author makes more than a dozen species out of forms, which are very
generally considered as varieties; and in this country the highest botanical authorities and practical men can be
quoted to show that the sessile and pedunculated oaks are either good and distinct species or mere varieties.
When a young naturalist commences the study of a group of organisms quite unknown to him, he is at first
much perplexed to determine what differences to consider as specific, and what as varieties; for he knows
nothing of the amount and kind of variation to which the group is subject; and this shows, at least, how very
generally there is some variation. But if he confine his attention to one class within one country, he will soon
make up his mind how to rank most of the doubtful forms. His general tendency will be to make many
species, for he will become impressed, just like the pigeon or poultry-fancier before alluded to, with the
amount of difference in the forms which he is continually studying; and he has little general knowledge of
analogical variation in other groups and in other countries, by which to correct his first impressions. As he
extends the range of his observations, he will meet with more cases of difficulty; for he will encounter a
greater number of closely-allied forms. But if his observations be widely extended, he will in the end
generally be enabled to make up his own mind which to call varieties and which species; but he will succeed
in this at the expense of admitting much variation,--and the truth of this admission will often be disputed by
other naturalists. When, moreover, he comes to study allied forms brought from countries not now continuous,
in which case he can hardly hope to find the intermediate links between his doubtful forms, he will have to
trust almost entirely to analogy, and his difficulties will rise to a climax.
Certainly no clear line of demarcation has as yet been drawn between species and sub-species--that is, the
forms which in the opinion of some naturalists come very near to, but do not quite arrive at the rank of
species; or, again, between sub-species and well-marked varieties, or between lesser varieties and individual
differences. These differences blend into each other in an insensible series; and a series impresses the mind
with the idea of an actual passage.
Hence I look at individual differences, though of small interest to the systematist, as of high importance for
us, as being the first step towards such slight varieties as are barely thought worth recording in works on
natural history. And I look at varieties which are in any degree more distinct and permanent, as steps leading
to more strongly marked and more permanent varieties; and at these latter, as leading to sub-species, and to
species. The passage from one stage of difference to another and higher stage may be, in some cases, due
merely to the long-continued action of different physical conditions in two different regions; but I have not
much faith in this view; and I attribute the passage of a variety, from a state in which it differs very slightly
from its parent to one in which it differs more, to the action of natural selection in accumulating (as will
hereafter be more fully explained) differences of structure in certain definite directions. Hence I believe a
well-marked variety may be justly called an incipient species; but whether this belief be justifiable must be
judged of by the general weight of the several facts and views given throughout this work.
It need not be supposed that all varieties or incipient species necessarily attain the rank of species. They may
whilst in this incipient state become extinct, or they may endure as varieties for very long periods, as has been
shown to be the case by Mr. Wollaston with the varieties of certain fossil land-shells in Madeira. If a variety
were to flourish so as to exceed in numbers the parent species, it would then rank as the species, and the
species as the variety; or it might come to supplant and exterminate the parent species; or both might co-exist,
and both rank as independent species. But we shall hereafter have to return to this subject.
From these remarks it will be seen that I look at the term species, as one arbitrarily given for the sake of
convenience to a set of individuals closely resembling each other, and that it does not essentially differ from
the term variety, which is given to less distinct and more fluctuating forms. The term variety, again, in
comparison with mere individual differences, is also applied arbitrarily, and for mere convenience sake.
Guided by theoretical considerations, I thought that some interesting results might be obtained in regard to the
nature and relations of the species which vary most, by tabulating all the varieties in several well-worked
floras. At first this seemed a simple task; but Mr. H. C. Watson, to whom I am much indebted for valuable
advice and assistance on this subject, soon convinced me that there were many difficulties, as did
subsequently Dr. Hooker, even in stronger terms. I shall reserve for my future work the discussion of these
difficulties, and the tables themselves of the proportional numbers of the varying species. Dr. Hooker permits
me to add, that after having carefully read my manuscript, and examined the tables, he thinks that the
following statements are fairly well established. The whole subject, however, treated as it necessarily here is
with much brevity, is rather perplexing, and allusions cannot be avoided to the "struggle for existence,"
"divergence of character," and other questions, hereafter to be discussed.
Alph. De Candolle and others have shown that plants which have very wide ranges generally present varieties;
and this might have been expected, as they become exposed to diverse physical conditions, and as they come
into competition (which, as we shall hereafter see, is a far more important circumstance) with different sets of
organic beings. But my tables further show that, in any limited country, the species which are most common,
that is abound most in individuals, and the species which are most widely diffused within their own country
(and this is a different consideration from wide range, and to a certain extent from commonness), often give
rise to varieties sufficiently well-marked to have been recorded in botanical works. Hence it is the most
flourishing, or, as they may be called, the dominant species,--those which range widely over the world, are the
most diffused in their own country, and are the most numerous in individuals,--which oftenest produce
well-marked varieties, or, as I consider them, incipient species. And this, perhaps, might have been
anticipated; for, as varieties, in order to become in any degree permanent, necessarily have to struggle with the
other inhabitants of the country, the species which are already dominant will be the most likely to yield
offspring which, though in some slight degree modified, will still inherit those advantages that enabled their
parents to become dominant over their compatriots.
If the plants inhabiting a country and described in any Flora be divided into two equal masses, all those in the
larger genera being placed on one side, and all those in the smaller genera on the other side, a somewhat larger
number of the very common and much diffused or dominant species will be found on the side of the larger
genera. This, again, might have been anticipated; for the mere fact of many species of the same genus
inhabiting any country, shows that there is something in the organic or inorganic conditions of that country
favourable to the genus; and, consequently, we might have expected to have found in the larger genera, or
those including many species, a large proportional number of dominant species. But so many causes tend to
obscure this result, that I am surprised that my tables show even a small majority on the side of the larger
genera. I will here allude to only two causes of obscurity. Fresh-water and salt-loving plants have generally
very wide ranges and are much diffused, but this seems to be connected with the nature of the stations
inhabited by them, and has little or no relation to the size of the genera to which the species belong. Again,
plants low in the scale of organisation are generally much more widely diffused than plants higher in the
scale; and here again there is no close relation to the size of the genera. The cause of lowly-organised plants
ranging widely will be discussed in our chapter on geographical distribution.
From looking at species as only strongly-marked and well-defined varieties, I was led to anticipate that the
species of the larger genera in each country would oftener present varieties, than the species of the smaller
genera; for wherever many closely related species (i.e. species of the same genus) have been formed, many
varieties or incipient species ought, as a general rule, to be now forming. Where many large trees grow, we
expect to find saplings. Where many species of a genus have been formed through variation, circumstances
have been favourable for variation; and hence we might expect that the circumstances would generally be still
favourable to variation. On the other hand, if we look at each species as a special act of creation, there is no
apparent reason why more varieties should occur in a group having many species, than in one having few.
To test the truth of this anticipation I have arranged the plants of twelve countries, and the coleopterous
insects of two districts, into two nearly equal masses, the species of the larger genera on one side, and those of
the smaller genera on the other side, and it has invariably proved to be the case that a larger proportion of the
species on the side of the larger genera present varieties, than on the side of the smaller genera. Moreover, the
species of the large genera which present any varieties, invariably present a larger average number of varieties
than do the species of the small genera. Both these results follow when another division is made, and when all
the smallest genera, with from only one to four species, are absolutely excluded from the tables. These facts
are of plain signification on the view that species are only strongly marked and permanent varieties; for
wherever many species of the same genus have been formed, or where, if we may use the expression, the
manufactory of species has been active, we ought generally to find the manufactory still in action, more
especially as we have every reason to believe the process of manufacturing new species to be a slow one. And
this certainly is the case, if varieties be looked at as incipient species; for my tables clearly show as a general
rule that, wherever many species of a genus have been formed, the species of that genus present a number of
varieties, that is of incipient species, beyond the average. It is not that all large genera are now varying much,
and are thus increasing in the number of their species, or that no small genera are now varying and increasing;
for if this had been so, it would have been fatal to my theory; inasmuch as geology plainly tells us that small
genera have in the lapse of time often increased greatly in size; and that large genera have often come to their
maxima, declined, and disappeared. All that we want to show is, that where many species of a genus have
been formed, on an average many are still forming; and this holds good.
There are other relations between the species of large genera and their recorded varieties which deserve
notice. We have seen that there is no infallible criterion by which to distinguish species and well-marked
varieties; and in those cases in which intermediate links have not been found between doubtful forms,
naturalists are compelled to come to a determination by the amount of difference between them, judging by
analogy whether or not the amount suffices to raise one or both to the rank of species. Hence the amount of
difference is one very important criterion in settling whether two forms should be ranked as species or
varieties. Now Fries has remarked in regard to plants, and Westwood in regard to insects, that in large genera
the amount of difference between the species is often exceedingly small. I have endeavoured to test this
numerically by averages, and, as far as my imperfect results go, they always confirm the view. I have also
consulted some sagacious and most experienced observers, and, after deliberation, they concur in this view. In
this respect, therefore, the species of the larger genera resemble varieties, more than do the species of the
smaller genera. Or the case may be put in another way, and it may be said, that in the larger genera, in which a
number of varieties or incipient species greater than the average are now manufacturing, many of the species
already manufactured still to a certain extent resemble varieties, for they differ from each other by a less than
usual amount of difference.
Moreover, the species of the large genera are related to each other, in the same manner as the varieties of any
one species are related to each other. No naturalist pretends that all the species of a genus are equally distinct
from each other; they may generally be divided into sub-genera, or sections, or lesser groups. As Fries has
well remarked, little groups of species are generally clustered like satellites around certain other species. And
what are varieties but groups of forms, unequally related to each other, and clustered round certain forms--that
is, round their parent-species? Undoubtedly there is one most important point of difference between varieties
and species; namely, that the amount of difference between varieties, when compared with each other or with
their parent-species, is much less than that between the species of the same genus. But when we come to
discuss the principle, as I call it, of Divergence of Character, we shall see how this may be explained, and how
the lesser differences between varieties will tend to increase into the greater differences between species.
There is one other point which seems to me worth notice. Varieties generally have much restricted ranges: this
statement is indeed scarcely more than a truism, for if a variety were found to have a wider range than that of
its supposed parent-species, their denominations ought to be reversed. But there is also reason to believe, that
those species which are very closely allied to other species, and in so far resemble varieties, often have much
restricted ranges. For instance, Mr. H. C. Watson has marked for me in the well-sifted London Catalogue of
plants (4th edition) 63 plants which are therein ranked as species, but which he considers as so closely allied
to other species as to be of doubtful value: these 63 reputed species range on an average over 6.9 of the
provinces into which Mr. Watson has divided Great Britain. Now, in this same catalogue, 53 acknowledged
varieties are recorded, and these range over 7.7 provinces; whereas, the species to which these varieties
belong range over 14.3 provinces. So that the acknowledged varieties have very nearly the same restricted
average range, as have those very closely allied forms, marked for me by Mr. Watson as doubtful species, but
which are almost universally ranked by British botanists as good and true species.
Finally, then, varieties have the same general characters as species, for they cannot be distinguished from
species,--except, firstly, by the discovery of intermediate linking forms, and the occurrence of such links
cannot affect the actual characters of the forms which they connect; and except, secondly, by a certain amount
of difference, for two forms, if differing very little, are generally ranked as varieties, notwithstanding that
intermediate linking forms have not been discovered; but the amount of difference considered necessary to
give to two forms the rank of species is quite indefinite. In genera having more than the average number of
species in any country, the species of these genera have more than the average number of varieties. In large
genera the species are apt to be closely, but unequally, allied together, forming little clusters round certain
species. Species very closely allied to other species apparently have restricted ranges. In all these several
respects the species of large genera present a strong analogy with varieties. And we can clearly understand
these analogies, if species have once existed as varieties, and have thus originated: whereas, these analogies
are utterly inexplicable if each species has been independently created.
We have, also, seen that it is the most flourishing and dominant species of the larger genera which on an
average vary most; and varieties, as we shall hereafter see, tend to become converted into new and distinct
species. The larger genera thus tend to become larger; and throughout nature the forms of life which are now
dominant tend to become still more dominant by leaving many modified and dominant descendants. But by
steps hereafter to be explained, the larger genera also tend to break up into smaller genera. And thus, the
forms of life throughout the universe become divided into groups subordinate to groups.
VARIATION UNDER NATURE.
Variability. Individual differences. Doubtful species. Wide ranging, much diffused, and common species vary
most. Species of the larger genera in any country vary more than the species of the smaller genera. Many of
the species of the larger genera resemble varieties in being very closely, but unequally, related to each other,
and in having restricted ranges.
Before applying the principles arrived at in the last chapter to organic beings in a state of nature, we must
briefly discuss whether these latter are subject to any variation. To treat this subject at all properly, a long
catalogue of dry facts should be given; but these I shall reserve for my future work. Nor shall I here discuss
the various definitions which have been given of the term species. No one definition has as yet satisfied all
naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. Generally the term
includes the unknown element of a distinct act of creation. The term "variety" is almost equally difficult to
define; but here community of descent is almost universally implied, though it can rarely be proved. We have
also what are called monstrosities; but they graduate into varieties. By a monstrosity I presume is meant some
considerable deviation of structure in one part, either injurious to or not useful to the species, and not
generally propagated. Some authors use the term "variation" in a technical sense, as implying a modification
directly due to the physical conditions of life; and "variations" in this sense are supposed not to be inherited:
but who can say that the dwarfed condition of shells in the brackish waters of the Baltic, or dwarfed plants on
Alpine summits, or the thicker fur of an animal from far northwards, would not in some cases be inherited for
at least some few generations? and in this case I presume that the form would be called a variety.
Again, we have many slight differences which may be called individual differences, such as are known
frequently to appear in the offspring from the same parents, or which may be presumed to have thus arisen,
from being frequently observed in the individuals of the same species inhabiting the same confined locality.
No one supposes that all the individuals of the same species are cast in the very same mould. These individual
differences are highly important for us, as they afford materials for natural selection to accumulate, in the
same manner as man can accumulate in any given direction individual differences in his domesticated
productions. These individual differences generally affect what naturalists consider unimportant parts; but I
could show by a long catalogue of facts, that parts which must be called important, whether viewed under a
physiological or classificatory point of view, sometimes vary in the individuals of the same species. I am
convinced that the most experienced naturalist would be surprised at the number of the cases of variability,
even in important parts of structure, which he could collect on good authority, as I have collected, during a
course of years. It should be remembered that systematists are far from pleased at finding variability in
important characters, and that there are not many men who will laboriously examine internal and important
organs, and compare them in many specimens of the same species. I should never have expected that the
branching of the main nerves close to the great central ganglion of an insect would have been variable in the
same species; I should have expected that changes of this nature could have been effected only by slow
degrees: yet quite recently Mr. Lubbock has shown a degree of variability in these main nerves in Coccus,
which may almost be compared to the irregular branching of the stem of a tree. This philosophical naturalist, I
may add, has also quite recently shown that the muscles in the larvae of certain insects are very far from
uniform. Authors sometimes argue in a circle when they state that important organs never vary; for these same
authors practically rank that character as important (as some few naturalists have honestly confessed) which
does not vary; and, under this point of view, no instance of an important part varying will ever be found: but
under any other point of view many instances assuredly can be given.
There is one point connected with individual differences, which seems to me extremely perplexing: I refer to
those genera which have sometimes been called "protean" or "polymorphic," in which the species present an
inordinate amount of variation; and hardly two naturalists can agree which forms to rank as species and which
as varieties. We may instance Rubus, Rosa, and Hieracium amongst plants, several genera of insects, and
CHAPTER 2.
VARIATION UNDER NATURE.
Variability. Individual differences. Doubtful species. Wide ranging, much diffused, and common species vary
most. Species of the larger genera in any country vary more than the species of the smaller genera. Many of
the species of the larger genera resemble varieties in being very closely, but unequally, related to each other,
and in having restricted ranges.
Before applying the principles arrived at in the last chapter to organic beings in a state of nature, we must
briefly discuss whether these latter are subject to any variation. To treat this subject at all properly, a long
catalogue of dry facts should be given; but these I shall reserve for my future work. Nor shall I here discuss
the various definitions which have been given of the term species. No one definition has as yet satisfied all
naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. Generally the term
includes the unknown element of a distinct act of creation. The term "variety" is almost equally difficult to
define; but here community of descent is almost universally implied, though it can rarely be proved. We have
also what are called monstrosities; but they graduate into varieties. By a monstrosity I presume is meant some
considerable deviation of structure in one part, either injurious to or not useful to the species, and not
generally propagated. Some authors use the term "variation" in a technical sense, as implying a modification
directly due to the physical conditions of life; and "variations" in this sense are supposed not to be inherited:
but who can say that the dwarfed condition of shells in the brackish waters of the Baltic, or dwarfed plants on
Alpine summits, or the thicker fur of an animal from far northwards, would not in some cases be inherited for
at least some few generations? and in this case I presume that the form would be called a variety.
Again, we have many slight differences which may be called individual differences, such as are known
frequently to appear in the offspring from the same parents, or which may be presumed to have thus arisen,
from being frequently observed in the individuals of the same species inhabiting the same confined locality.
No one supposes that all the individuals of the same species are cast in the very same mould. These individual
differences are highly important for us, as they afford materials for natural selection to accumulate, in the
same manner as man can accumulate in any given direction individual differences in his domesticated
productions. These individual differences generally affect what naturalists consider unimportant parts; but I
could show by a long catalogue of facts, that parts which must be called important, whether viewed under a
physiological or classificatory point of view, sometimes vary in the individuals of the same species. I am
convinced that the most experienced naturalist would be surprised at the number of the cases of variability,
even in important parts of structure, which he could collect on good authority, as I have collected, during a
course of years. It should be remembered that systematists are far from pleased at finding variability in
important characters, and that there are not many men who will laboriously examine internal and important
organs, and compare them in many specimens of the same species. I should never have expected that the
branching of the main nerves close to the great central ganglion of an insect would have been variable in the
same species; I should have expected that changes of this nature could have been effected only by slow
degrees: yet quite recently Mr. Lubbock has shown a degree of variability in these main nerves in Coccus,
which may almost be compared to the irregular branching of the stem of a tree. This philosophical naturalist, I
may add, has also quite recently shown that the muscles in the larvae of certain insects are very far from
uniform. Authors sometimes argue in a circle when they state that important organs never vary; for these same
authors practically rank that character as important (as some few naturalists have honestly confessed) which
does not vary; and, under this point of view, no instance of an important part varying will ever be found: but
under any other point of view many instances assuredly can be given.
There is one point connected with individual differences, which seems to me extremely perplexing: I refer to
those genera which have sometimes been called "protean" or "polymorphic," in which the species present an
inordinate amount of variation; and hardly two naturalists can agree which forms to rank as species and which
as varieties. We may instance Rubus, Rosa, and Hieracium amongst plants, several genera of insects, and
be sufficient? It must be admitted that many forms, considered by highly-competent judges as varieties, have
so perfectly the character of species that they are ranked by other highly-competent judges as good and true
species. But to discuss whether they are rightly called species or varieties, before any definition of these terms
has been generally accepted, is vainly to beat the air.
Many of the cases of strongly-marked varieties or doubtful species well deserve consideration; for several
interesting lines of argument, from geographical distribution, analogical variation, hybridism, etc., have been
brought to bear on the attempt to determine their rank. I will here give only a single instance,--the well-known
one of the primrose and cowslip, or Primula veris and elatior. These plants differ considerably in appearance;
they have a different flavour and emit a different odour; they flower at slightly different periods; they grow in
somewhat different stations; they ascend mountains to different heights; they have different geographical
ranges; and lastly, according to very numerous experiments made during several years by that most careful
observer Gartner, they can be crossed only with much difficulty. We could hardly wish for better evidence of
the two forms being specifically distinct. On the other hand, they are united by many intermediate links, and it
is very doubtful whether these links are hybrids; and there is, as it seems to me, an overwhelming amount of
experimental evidence, showing that they descend from common parents, and consequently must be ranked as
varieties.
Close investigation, in most cases, will bring naturalists to an agreement how to rank doubtful forms. Yet it
must be confessed, that it is in the best-known countries that we find the greatest number of forms of doubtful
value. I have been struck with the fact, that if any animal or plant in a state of nature be highly useful to man,
or from any cause closely attract his attention, varieties of it will almost universally be found recorded. These
varieties, moreover, will be often ranked by some authors as species. Look at the common oak, how closely it
has been studied; yet a German author makes more than a dozen species out of forms, which are very
generally considered as varieties; and in this country the highest botanical authorities and practical men can be
quoted to show that the sessile and pedunculated oaks are either good and distinct species or mere varieties.
When a young naturalist commences the study of a group of organisms quite unknown to him, he is at first
much perplexed to determine what differences to consider as specific, and what as varieties; for he knows
nothing of the amount and kind of variation to which the group is subject; and this shows, at least, how very
generally there is some variation. But if he confine his attention to one class within one country, he will soon
make up his mind how to rank most of the doubtful forms. His general tendency will be to make many
species, for he will become impressed, just like the pigeon or poultry-fancier before alluded to, with the
amount of difference in the forms which he is continually studying; and he has little general knowledge of
analogical variation in other groups and in other countries, by which to correct his first impressions. As he
extends the range of his observations, he will meet with more cases of difficulty; for he will encounter a
greater number of closely-allied forms. But if his observations be widely extended, he will in the end
generally be enabled to make up his own mind which to call varieties and which species; but he will succeed
in this at the expense of admitting much variation,--and the truth of this admission will often be disputed by
other naturalists. When, moreover, he comes to study allied forms brought from countries not now continuous,
in which case he can hardly hope to find the intermediate links between his doubtful forms, he will have to
trust almost entirely to analogy, and his difficulties will rise to a climax.
Certainly no clear line of demarcation has as yet been drawn between species and sub-species--that is, the
forms which in the opinion of some naturalists come very near to, but do not quite arrive at the rank of
species; or, again, between sub-species and well-marked varieties, or between lesser varieties and individual
differences. These differences blend into each other in an insensible series; and a series impresses the mind
with the idea of an actual passage.
Hence I look at individual differences, though of small interest to the systematist, as of high importance for
us, as being the first step towards such slight varieties as are barely thought worth recording in works on
natural history. And I look at varieties which are in any degree more distinct and permanent, as steps leading
to more strongly marked and more permanent varieties; and at these latter, as leading to sub-species, and to
species. The passage from one stage of difference to another and higher stage may be, in some cases, due
merely to the long-continued action of different physical conditions in two different regions; but I have not
much faith in this view; and I attribute the passage of a variety, from a state in which it differs very slightly
from its parent to one in which it differs more, to the action of natural selection in accumulating (as will
hereafter be more fully explained) differences of structure in certain definite directions. Hence I believe a
well-marked variety may be justly called an incipient species; but whether this belief be justifiable must be
judged of by the general weight of the several facts and views given throughout this work.
It need not be supposed that all varieties or incipient species necessarily attain the rank of species. They may
whilst in this incipient state become extinct, or they may endure as varieties for very long periods, as has been
shown to be the case by Mr. Wollaston with the varieties of certain fossil land-shells in Madeira. If a variety
were to flourish so as to exceed in numbers the parent species, it would then rank as the species, and the
species as the variety; or it might come to supplant and exterminate the parent species; or both might co-exist,
and both rank as independent species. But we shall hereafter have to return to this subject.
From these remarks it will be seen that I look at the term species, as one arbitrarily given for the sake of
convenience to a set of individuals closely resembling each other, and that it does not essentially differ from
the term variety, which is given to less distinct and more fluctuating forms. The term variety, again, in
comparison with mere individual differences, is also applied arbitrarily, and for mere convenience sake.
Guided by theoretical considerations, I thought that some interesting results might be obtained in regard to the
nature and relations of the species which vary most, by tabulating all the varieties in several well-worked
floras. At first this seemed a simple task; but Mr. H. C. Watson, to whom I am much indebted for valuable
advice and assistance on this subject, soon convinced me that there were many difficulties, as did
subsequently Dr. Hooker, even in stronger terms. I shall reserve for my future work the discussion of these
difficulties, and the tables themselves of the proportional numbers of the varying species. Dr. Hooker permits
me to add, that after having carefully read my manuscript, and examined the tables, he thinks that the
following statements are fairly well established. The whole subject, however, treated as it necessarily here is
with much brevity, is rather perplexing, and allusions cannot be avoided to the "struggle for existence,"
"divergence of character," and other questions, hereafter to be discussed.
Alph. De Candolle and others have shown that plants which have very wide ranges generally present varieties;
and this might have been expected, as they become exposed to diverse physical conditions, and as they come
into competition (which, as we shall hereafter see, is a far more important circumstance) with different sets of
organic beings. But my tables further show that, in any limited country, the species which are most common,
that is abound most in individuals, and the species which are most widely diffused within their own country
(and this is a different consideration from wide range, and to a certain extent from commonness), often give
rise to varieties sufficiently well-marked to have been recorded in botanical works. Hence it is the most
flourishing, or, as they may be called, the dominant species,--those which range widely over the world, are the
most diffused in their own country, and are the most numerous in individuals,--which oftenest produce
well-marked varieties, or, as I consider them, incipient species. And this, perhaps, might have been
anticipated; for, as varieties, in order to become in any degree permanent, necessarily have to struggle with the
other inhabitants of the country, the species which are already dominant will be the most likely to yield
offspring which, though in some slight degree modified, will still inherit those advantages that enabled their
parents to become dominant over their compatriots.
If the plants inhabiting a country and described in any Flora be divided into two equal masses, all those in the
larger genera being placed on one side, and all those in the smaller genera on the other side, a somewhat larger
number of the very common and much diffused or dominant species will be found on the side of the larger
genera. This, again, might have been anticipated; for the mere fact of many species of the same genus
inhabiting any country, shows that there is something in the organic or inorganic conditions of that country
favourable to the genus; and, consequently, we might have expected to have found in the larger genera, or
those including many species, a large proportional number of dominant species. But so many causes tend to
obscure this result, that I am surprised that my tables show even a small majority on the side of the larger
genera. I will here allude to only two causes of obscurity. Fresh-water and salt-loving plants have generally
very wide ranges and are much diffused, but this seems to be connected with the nature of the stations
inhabited by them, and has little or no relation to the size of the genera to which the species belong. Again,
plants low in the scale of organisation are generally much more widely diffused than plants higher in the
scale; and here again there is no close relation to the size of the genera. The cause of lowly-organised plants
ranging widely will be discussed in our chapter on geographical distribution.
From looking at species as only strongly-marked and well-defined varieties, I was led to anticipate that the
species of the larger genera in each country would oftener present varieties, than the species of the smaller
genera; for wherever many closely related species (i.e. species of the same genus) have been formed, many
varieties or incipient species ought, as a general rule, to be now forming. Where many large trees grow, we
expect to find saplings. Where many species of a genus have been formed through variation, circumstances
have been favourable for variation; and hence we might expect that the circumstances would generally be still
favourable to variation. On the other hand, if we look at each species as a special act of creation, there is no
apparent reason why more varieties should occur in a group having many species, than in one having few.
To test the truth of this anticipation I have arranged the plants of twelve countries, and the coleopterous
insects of two districts, into two nearly equal masses, the species of the larger genera on one side, and those of
the smaller genera on the other side, and it has invariably proved to be the case that a larger proportion of the
species on the side of the larger genera present varieties, than on the side of the smaller genera. Moreover, the
species of the large genera which present any varieties, invariably present a larger average number of varieties
than do the species of the small genera. Both these results follow when another division is made, and when all
the smallest genera, with from only one to four species, are absolutely excluded from the tables. These facts
are of plain signification on the view that species are only strongly marked and permanent varieties; for
wherever many species of the same genus have been formed, or where, if we may use the expression, the
manufactory of species has been active, we ought generally to find the manufactory still in action, more
especially as we have every reason to believe the process of manufacturing new species to be a slow one. And
this certainly is the case, if varieties be looked at as incipient species; for my tables clearly show as a general
rule that, wherever many species of a genus have been formed, the species of that genus present a number of
varieties, that is of incipient species, beyond the average. It is not that all large genera are now varying much,
and are thus increasing in the number of their species, or that no small genera are now varying and increasing;
for if this had been so, it would have been fatal to my theory; inasmuch as geology plainly tells us that small
genera have in the lapse of time often increased greatly in size; and that large genera have often come to their
maxima, declined, and disappeared. All that we want to show is, that where many species of a genus have
been formed, on an average many are still forming; and this holds good.
There are other relations between the species of large genera and their recorded varieties which deserve
notice. We have seen that there is no infallible criterion by which to distinguish species and well-marked
varieties; and in those cases in which intermediate links have not been found between doubtful forms,
naturalists are compelled to come to a determination by the amount of difference between them, judging by
analogy whether or not the amount suffices to raise one or both to the rank of species. Hence the amount of
difference is one very important criterion in settling whether two forms should be ranked as species or
varieties. Now Fries has remarked in regard to plants, and Westwood in regard to insects, that in large genera
the amount of difference between the species is often exceedingly small. I have endeavoured to test this
numerically by averages, and, as far as my imperfect results go, they always confirm the view. I have also
consulted some sagacious and most experienced observers, and, after deliberation, they concur in this view. In
this respect, therefore, the species of the larger genera resemble varieties, more than do the species of the
smaller genera. Or the case may be put in another way, and it may be said, that in the larger genera, in which a
number of varieties or incipient species greater than the average are now manufacturing, many of the species
already manufactured still to a certain extent resemble varieties, for they differ from each other by a less than
usual amount of difference.
Moreover, the species of the large genera are related to each other, in the same manner as the varieties of any
one species are related to each other. No naturalist pretends that all the species of a genus are equally distinct
from each other; they may generally be divided into sub-genera, or sections, or lesser groups. As Fries has
well remarked, little groups of species are generally clustered like satellites around certain other species. And
what are varieties but groups of forms, unequally related to each other, and clustered round certain forms--that
is, round their parent-species? Undoubtedly there is one most important point of difference between varieties
and species; namely, that the amount of difference between varieties, when compared with each other or with
their parent-species, is much less than that between the species of the same genus. But when we come to
discuss the principle, as I call it, of Divergence of Character, we shall see how this may be explained, and how
the lesser differences between varieties will tend to increase into the greater differences between species.
There is one other point which seems to me worth notice. Varieties generally have much restricted ranges: this
statement is indeed scarcely more than a truism, for if a variety were found to have a wider range than that of
its supposed parent-species, their denominations ought to be reversed. But there is also reason to believe, that
those species which are very closely allied to other species, and in so far resemble varieties, often have much
restricted ranges. For instance, Mr. H. C. Watson has marked for me in the well-sifted London Catalogue of
plants (4th edition) 63 plants which are therein ranked as species, but which he considers as so closely allied
to other species as to be of doubtful value: these 63 reputed species range on an average over 6.9 of the
provinces into which Mr. Watson has divided Great Britain. Now, in this same catalogue, 53 acknowledged
varieties are recorded, and these range over 7.7 provinces; whereas, the species to which these varieties
belong range over 14.3 provinces. So that the acknowledged varieties have very nearly the same restricted
average range, as have those very closely allied forms, marked for me by Mr. Watson as doubtful species, but
which are almost universally ranked by British botanists as good and true species.
Finally, then, varieties have the same general characters as species, for they cannot be distinguished from
species,--except, firstly, by the discovery of intermediate linking forms, and the occurrence of such links
cannot affect the actual characters of the forms which they connect; and except, secondly, by a certain amount
of difference, for two forms, if differing very little, are generally ranked as varieties, notwithstanding that
intermediate linking forms have not been discovered; but the amount of difference considered necessary to
give to two forms the rank of species is quite indefinite. In genera having more than the average number of
species in any country, the species of these genera have more than the average number of varieties. In large
genera the species are apt to be closely, but unequally, allied together, forming little clusters round certain
species. Species very closely allied to other species apparently have restricted ranges. In all these several
respects the species of large genera present a strong analogy with varieties. And we can clearly understand
these analogies, if species have once existed as varieties, and have thus originated: whereas, these analogies
are utterly inexplicable if each species has been independently created.
We have, also, seen that it is the most flourishing and dominant species of the larger genera which on an
average vary most; and varieties, as we shall hereafter see, tend to become converted into new and distinct
species. The larger genera thus tend to become larger; and throughout nature the forms of life which are now
dominant tend to become still more dominant by leaving many modified and dominant descendants. But by
steps hereafter to be explained, the larger genera also tend to break up into smaller genera. And thus, the
forms of life throughout the universe become divided into groups subordinate to groups.
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