Chapter II Variation Under
Nature
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.
2 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.
3 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.
4 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.
5 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.
6 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.
7 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 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.
8 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.
9 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.
10 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.
11 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.
12 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.
13 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.
14 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.
15 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.
16 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.
17 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.
18 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.
19 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.
20 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.
21 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.
22 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.
23 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.