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.

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 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, &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.

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.