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 larvæ 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 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 judgment 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, 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 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 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.