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kind ever do appear in a state of nature and are capable of reproduction
(which is not always the case) , as they occur rarely and singly, their preserva-
tion would depend on unusually favorable circumstances. They would, also,
during the first and succeeding generations, cross with the ordinary form,

27



THE ORIGIN OF SPECIES

and thus their abnormal character would almost inevitably be lost. But I shall
have to return in a future chapter to the preservation and perpetuation of
single or occasional variations.

INDIVIDUAL DIFFERENCES

The many slight difTerences which appear in the offspring from the same
parents, or which it may be presumed have thus arisen, from being observed
in the individuals of the same species inhabiting the same confined locality,
may be called individual differences. No one supposes that all the individuals
of the same species are cast in the same actual mould. These individual dif-
ferences are of the highest importance for us, for they are often inherited, as
must be familiar to every one; and they thus afford materials for natural
selection to act on and accumulate, in the same manner as man accumulates
in any given direction individual differences in his domesticated productions.
These individual differences generally affect what naturalists consider unim-
portant parts ; but I could show, by a long catalogue of facts, that parts which
must be called important, whether viewed under a physiological or classifica-
tory 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 he could collect on good authority, as I have collected,
during a course of years. It should be remembered that systematists are far
from being 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. It would
never have been expected that the branching of the main nerves close to the
great central ganglion of an insect would have been variable in the same spe-
cies ; it might have been thought that changes of this nature could have been
effected only by slow degrees ; yet Sir J. Lubbock has shown a degree of var-
iability 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 shown that the muscles in the larvae of certain insects
are far from uniform. Authors sometimes argue in a circle when they state
that important organs never vary; for these same authors practically rank
those parts as important (as some few naturalists have honestly confessed)
which do not vary; and, under this point of view, no instance will ever be
found of an important part varying; but under any other point of view many
instances assuredly can be given.

There is one point connected with individual differences which is extremely
perplexing: I refer to those genera which have been called "protean" or
"polymorphic," in which species present an inordinate amount of variation.
With respect to many of these forms, hardly two naturalists agree whether
to rank them as species or as varieties. We may instance Rubus, Rosa, and
Hieracium among plants, several genera of insects and of Brachiopod shells.
In most polymorphic genera some of the species have fixed and definite char-
acters. Genera which are polymorphic in one country seem to be, with a few



VARIATION UNDER NATURE 29

exceptions, polymorphic in other countries, and likewise, judging from Bra-
chiopod shells, at former periods of time. These facts are very perplexing, for
they seem to show that this kind of variabiHty is independent of the condi-
tions of life. I am inclined to suspect that we see, at least in some of these
polymorphic genera, variations which are of no service or disservice to the
species, and which consequently have not been seized on and rendered defi-
nite by natural selection, as hereafter to be explained.

Individuals of the same species often present, as is known to every one,
great differences of structure, independently of variation, as in the two sexes
of various animals, in the two or three castes of sterile females or workers
among insects, and in the immature and larval states of many of the lower
animals. There are, also, cases of dimorphism and trimorphism, both with
animals and plants. Thus, Mr. Wallace, who has lately called attention to
the subject, has shown that the females of certain species of butterflies, in the
Malayan Archipelago, regularly appeared under two or even three conspicu-
ously distinct forms, not connected by intermediate varieties. Fritz Miiller
has described analogous but more extraordinary cases with the males of cer-
tain Brazilian Crustaceans : thus, the male of a Tanais regularly occurs under
two distinct forms; one of these has strong and differently shaped pincers,
and the other has antennae much more abundantly furnished with smelHng-
hairs. Although in most of these cases, the two or three forms, both with ani-
mals and plants, are not now connected by intermediate gradations, it is
probable that they were once thus connected. Mr. Wallace, for instance,
describes a certain butterfly which presents in the same island a great range
of varieties connected by intermediate links, and the extreme links of the
chain closely resemble the two forms of an allied dimorphic species inhabiting
another part of the Malay Archipelago. Thus also with ants, the several
worker-castes are generally quite distinct; but in some cases, as we shall here-
after see, the castes are connected together by finely graduated varieties. So
it is, as I have myself observed, with some dimorphic plants. It certainly at
first appears a highly remarkable fact that the same female butterfly should
have the power of producing at the same time three distinct female forms and
a male; and that an hermaphrodite plant should produce from the same
seed-capsule three distinct hermaphrodite forms, bearing three different kinds
of females and three or even six different kinds of males. Nevertheless these
cases are only exaggerations of the common fact that the female produces
offspring of two sexes which sometimes differ from each other in a wonderful
manner.

DOUBTFUL SPECIES

The forms which possess in some considerable degree the character of spe-
cies, but which are so closely similar to 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 for a long time; for as long, as



30 THE ORIGIN OF SPECIES

far as we know, as have good and true species. Practically, when a naturalist
can unite by means of intermediate links any two forms, 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 diffi-
culty, which I will not here enumerate, sometimes arise 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 forms 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 experi-
ence 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 com-
petent 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 obli-
gation 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 va-
rieties, 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!
Among 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 the 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, geographical races! Mr. Wallace, in several valuable
papers on the various animals, especially on the Lepidoptera, inhabiting the
islands of the great Malayan Archipelago, shows that they may be classed
under four heads, namely, as variable forms, as local forms, as geographical
races or sub-species, and as true representative species. The first or variable
forms vary much within the limits of the same island. The local forms are
moderately constant and distinct in each separate island; but when all from
the several islands are compared together, the differences are seen to be so



VARIATION UNDER NATURE 31

slight and graduated that it is impossible to define or describe them, though
at the same time the extreme forms are sufficiently distinct. The geographical
races or sub-species are local forms completely fixed and isolated ; but as they
do not differ from each other by strongly marked and important characters,
"There is no possible test but individual opinion to determine which of them
shall be considered as species and which as varieties." Lastly, representative
species fill the same place in the natural economy of each island as do the
local forms and sub-species ; but as they are distinguished from each other by
sa greater amount of difference than that between the local forms and sub-
species, they are almost universally ranked by naturalists as true species.
Nevertheless, no certain criterion can possibly be given by which variable
forms, local forms, sub-species, and representative species can be recognized.

Many years ago, when comparing, and seeing others compare, the birds
from the closely neighboring islands of the Galapagos Archipelago, 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 varie-
ties. 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
would certainly 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 experienced ornithol-
ogists consider our British red grouse as only a strongly marked race of a
Norwegian species, whereas the greater number rank it as an undoubted spe-
cies peculiar to Great Britain. A wide distance between the homes of two
doubtful forms leads many naturalists to rank them as distinct species; but
what distance, it has been well asked, will suffice if that between America
and Europe is ample? will that between Europe and the Azores, or Madeira,
or the Canaries, or between the several islets of these small archipelagoes, be
sufficient?

Mr. B. D. Walsh, a distinguished entomologist of the United States, has
described what he calls Phytophagic varieties and Phytophagic species. Most
vegetable-feeding insects live on one kind of plant or on one group of plants;
some feed indiscriminately on many kinds, but do not in consequence vary.
In several cases, however, insects found living on different plants, have been
observed by Mr. Walsh to present in their larval or mature state, or in both
states, slight though constant differences in color, size, or in the nature of their
secretions. In some instances the males alone, in other instances both males
and females, have been observed thus to differ in a slight degree. When the
differences are rather more strongly marked, and when both sexes and all ages
are affected, the forms are ranked by all entomologists as good species. But
no observer can determine for another, even if he can do so for himself,
which of these Phytophagic forms ought to be called species and which varie-
ties. Mr. Walsh ranks the forms which it may be supposed would freely inter-
cross, as varieties; and those which appear to have lost this power, as species.
As the differences depend on the insects having long fed on distinct plants, it



32 THE ORIGIN OF SPECIES

cannot be expected that intermediate links connecting the several forms
should now be found. The naturalist thus loses his best guide in determining
whether to rank doubtful forms as varieties or species. This likewise neces-
sarily occurs with closely allied organisms, which inhabit distinct continents
or islands. When, on the other hand, an animal or plant ranges over the same
continent, or inhabits many islands in the same archipelago, and presents dif-
ferent forms in the different areas, there is always a good chance that inter-
mediate forms will be discovered which will link together the extreme states;
and these are then degraded to the rank of varieties.

Some few naturalists maintain that animals never present varieties; but
then these same naturalists rank the slightest difference as of specific value;
and when the same identical form is met with in two distant countries, or in
two geological formations, they believe that two distinct species are hidden
under the same dress. The term species thus comes to be a mere useless ab-
straction, implying and assuming a separate act of creation. It is certain that
many forms, considered by highly competent judges to be varieties, resemble
species so completely in character that they have been thus ranked by other
highly competent judges. But to discuss whether they ought to be 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 de-
serve consideration; for several interesting lines of argument, from geograph-
ical distribution, analogical variation, hybridism, etc., have been brought to
bear in the attempt to determine their rank; but space does not here permit
me to discuss them. Close investigation, in many cases, will no doubt bring
naturalists to agree 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
them. 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 attracts his at-
tention, varieties of it will almost universally be found recorded. These varie-
ties, moreover, will often be 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 almost universally con-
sidered by other botanists to be varieties ; and in this country the highest bo-
tanical authorities and practical men can be quoted to show that the sessile
and pedunculated oaks are either good and distinct species or mere varieties.

I may here allude to a remarkable memoir lately published by A. de Can-
dolle, on the oaks of the whole world. No one ever had more ample materials
for the discrimination of the species, or could have worked on them with
more zeal and sagacity. He first gives in detail all the many points of struc-
ture which vary in the several species, and estimates numerically the relative
frequency of the variations. He specifies above a dozen characters which may
be found varying even on the same branch, sometimes according to age or
development, sometimes without any assignable reason. Such characters are
not of course of specific value, but they are, as Asa Gray has remarked in
commenting on this memoir, such as generally enter into specific definitions.



VARIATION UNDER NATURE 33

De Candolle then goes on to say that he gives the rank of species to the forms
that differ by characters never varying on the same tree, and never found
connected by intermediate states. After this discussion, the result of so much
labor, he emphatically remarks: "They are mistaken, who repeat that the
greater part of our species are clearly limited, and that the doubtful species
are in a feeble minority. This seemed to be true, so long as a genus was im-
perfectly known, and its species were founded upon a few specimens, that is
to say, were provisional. Just as we come to know them better, intermediate
forms flow in, and doubts as to specific limits augment." He also adds that it
is the best-known species which present the greatest number of spontaneous
varieties and sub-varieties. Thus Quercus robur has twenty-eight varieties, all
of which, excepting six, are clustered round three sub-species, namely, Q.
pedunculata, sessiliflora, and pubescens. The forms which connect these three
sub-species are comparatively rare; and, as Asa Gray again remarks, if these
connecting forms which are now rare were to become totally extinct, the
three sub-species would hold exactly the same relation to each other as do
the four or five provisionally admitted species which closely surround the
typical Quercus robur. Finally, De Candolle admits that out of the 300 spe-
cies, which will be enumerated in his Prodromus as belonging to the oak
family, at least two-thirds are provisional species, that is, are not known
strictly to fulfil the definition above given of a true species. It should be added
that De Candolle no longer believes that species are immutable creations, but
concludes that the derivative theory is the most natural one, "and the most
accordant with the known facts in palaeontology, geographical botany, and
zoology, of anatomical structure and classification."

When a young naturalist commences the study of a group of organisms
quite unknown to him, he is at first much perplexed in determining what
differences to consider as specific and what as varietal ; 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 poul-
try fancier before alluded to, with the amount of difference in the forms
which he is continually studying; and he has little general knowledge of ana-
logical 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 able to make up his own mind ; 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 he comes to study allied forms
brought from countries not now continuous, in which case he cannot hope to
find intermediate links, he will be compelled to trust almost entirely to anal-
ogy, and his difficulties will rise to a climax.

Certainly no clear line of demarcation has as yet been drawn between spe-



34 THE ORIGIN OF SPECIES

cies and sub-species — that is, the forms which in the opinion of some natu-
ralists 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 varie-
ties and individual differences. These differences blend into each other by 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 sys-
tematist, as of the highest importance for us, as being the first steps toward
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 toward more strongly marked and permanent varieties;
and at the latter, as leading to sub-species and then to species. The passage
from one stage of difference to another may, in some cases, be the simple
result of the nature of the organism and of the different physical conditions
to which it has long been exposed; but with respect to the more important
and adaptive characters, the passage from one stage of difference to another
may be safely attributed to the cumulative action of natural selection, here-
after to be explained, and to the effects of the increased use or disuse of parts.
A well-marked variety may therefore be called an incipient species; but
whether this belief is justifiable must be judged by the weight of the various
facts and considerations to be given throughout this work.

It need not be supposed that all varieties or incipient species attain the
rank of species. They may become extinct, or they may endure as varieties
for very long periods, as has been shown to be the case by Mr. WoUaston
with the varieties of certain fossil land-shells in Madeira, and with plants by
Gaston de Saporta. 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 here-
after 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, for convenience' sake.

WroE-RANGING, MUCH DIFFUSED, AND COMMON SPECIES VARY MOST

Guided by theoretical considerations, I thought that some interesting re-
sults 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 con-
vinced me that there were many difficulties, as did subsequently Dr. Hooper,
even in stronger terms. I shall reserve for a future work the discussion of these y



VARIATION UNDER NATURE 35

difficulties, and the tables of the proportional numbers of the varying species.
Dr. Hooper permits me to add that after having carefully read my manu-
script, 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.

Alphonso de Candolle and others have shown that plants which have very
wide ranges generally present varieties; and this might have been expected,
as they are exposed to diverse physical conditions, and as they come into com-
petition (which, as we shall hereafter see, is an equally or more important
circumstance) with different sets of organic beings. But my tables further



Online LibraryCharles DarwinThe origin of species → online text (page 5 of 50)