The Origin of Species
Chapter 2: Variation Under Nature
by Charles Darwin
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 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.