On the Origin of Species by Means of Natural Selection (1859)
by Charles Darwin
CHAPTER II.
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.
EFORE 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 any 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 several genera of Brachiopod shells. In most
polymorphic genera some of the species have fixed and definite
characters. Genera which are polymorphic in one country seem to be,
with some few exceptions, polymorphic in other countries, and
likewise, judging from Brachiopod shells, at former periods of time.
These facts seem to be very perplexing, for they seem to show that
this kind of variability is independent of the conditions of life. I
am inclined to suspect that we see in these polymorphic genera
variations in points of structure which are of no service or
disservice to the species, and which consequently have not been seized
on and rendered definite by natural selection, as hereafter will be
explained.
Those forms which possess in some considerable degree the character of
species, but which are so closely similar to some other forms, or are
so closely linked to them by intermediate gradations, that naturalists
do not like to rank them as distinct species, are in several respects
the most important for us. We have every reason to believe that many
of these doubtful and closely-allied forms have permanently retained
their characters in their own country for a long time; for as long, as
far as we know, as have good and true species. practically, when a
naturalist can unite two forms together by others having intermediate
characters, he treats the one as a variety of the other, ranking the
most common, but sometimes the one first described, as the species,
and the other as the variety. But cases of great difficulty, which I
will not here enumerate, sometimes occur in deciding whether or not to
rank one form as a variety of another, even when they are closely
connected by intermediate links; nor will the commonly-assumed hybrid
nature of the intermediate links always remove the difficulty. In very
many cases, however, one form is ranked as a variety of another, not
because the intermediate links have actually been found, but because
analogy leads the observer to suppose either that they do now
somewhere exist, or may formerly have existed; and here a wide door
for the entry of doubt and conjecture is opened.
Hence, in determining whether a form should be ranked as a species or
a variety, the opinion of naturalists having sound judgement and wide
experience seems the only guide to follow. We must, however, in many
cases, decide by a majority of naturalists, for few well-marked and
well-known varieties can be named which have not been ranked as
species by at least some competent judges.
That varieties of this doubtful nature are far from uncommon cannot be
disputed. Compare the several floras of Great Britain, of France or of
the United States, drawn up by different botanists, and see what a
surprising number of forms have been ranked by one botanist as good
species, and by another as mere varieties. Mr. H. C. Watson, to whom I
lie under deep obligation for assistance of all kinds, has marked for
me 182 British plants, which are generally considered as varieties,
but which have all been ranked by botanists as species; and in making
this list he has omitted many trifling varieties, but which
nevertheless have been ranked by some botanists as species, and he has
entirely omitted several highly polymorphic genera. Under genera,
including the most polymorphic forms, Mr. Babington gives 251 species,
whereas Mr. Bentham gives only 112,—a difference of 139 doubtful forms!
Amongst animals which unite for each birth, and which are highly
locomotive, doubtful forms, ranked by one zoologist as a species and
by another as a variety, can rarely be found within the same country,
but are common in separated areas. How many of those birds and insects
in North America and Europe, which differ very slightly from each
other, have been ranked by one eminent naturalist as undoubted
species, and by another as varieties, or, as they are often called, as
geographical races! Many years ago, when comparing, and seeing others
compare, the birds from the separate islands of the Galapagos
Archipelago, both one with another, and with those from the American
mainland, I was much struck how entirely vague and arbitrary is the
distinction between species and varieties. On the islets of the little
Madeira group there are many insects which are characterized as
varieties in Mr. Wollaston's admirable work, but which it cannot be
doubted would be ranked as distinct species by many
entomologists. Even Ireland has a few animals, now generally regarded
as varieties, but which have been ranked as species by some
zoologists. Several most experienced ornithologists consider our
British red grouse as only a strongly-marked race of a Norwegian
species, whereas the greater number rank it as an undoubted species
peculiar to Great Britain. A wide distance between the homes of two
doubtful forms leads many naturalists to rank both as distinct
species; but what distance, it has been well asked, will suffice? if
that between America and Europe is ample, will that between the
Continent and the Azores, or Madeira, or the Canaries, or Ireland, 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,
&c., 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 Gärtner, 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 whenever 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.
[ Charles Darwin,
On
the Origin Of Species: A Facsimile of the First Edition,
Cambridge, Massachusetts: Harvard University Press, 1964, pp. 44-59. ]
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