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Books: On the Origin of Species

C >> Charles Darwin >> On the Origin of Species




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.


CHAPTER 3. STRUGGLE FOR EXISTENCE.

Bears on natural selection.
The term used in a wide sense.
Geometrical powers of increase.
Rapid increase of naturalised animals and plants.
Nature of the checks to increase.
Competition universal.
Effects of climate.
Protection from the number of individuals.
Complex relations of all animals and plants throughout nature.
Struggle for life most severe between individuals and varieties of the
same species; often severe between species of the same genus.
The relation of organism to organism the most important of all
relations.

Before entering on the subject of this chapter, I must make a few
preliminary remarks, to show how the struggle for existence bears on
Natural Selection. It has been seen in the last chapter that amongst
organic beings in a state of nature there is some individual
variability; indeed I am not aware that this has ever been disputed.
It is immaterial for us whether a multitude of doubtful forms be
called species or sub-species or varieties; what rank, for instance,
the two or three hundred doubtful forms of British plants are entitled
to hold, if the existence of any well-marked varieties be admitted.
But the mere existence of individual variability and of some few
well-marked varieties, though necessary as the foundation for the
work, helps us but little in understanding how species arise in
nature. How have all those exquisite adaptations of one part of the
organisation to another part, and to the conditions of life, and of
one distinct organic being to another being, been perfected? We see
these beautiful co-adaptations most plainly in the woodpecker and
missletoe; and only a little less plainly in the humblest parasite
which clings to the hairs of a quadruped or feathers of a bird; in the
structure of the beetle which dives through the water; in the plumed
seed which is wafted by the gentlest breeze; in short, we see
beautiful adaptations everywhere and in every part of the organic
world.

Again, it may be asked, how is it that varieties, which I have called
incipient species, become ultimately converted into good and distinct
species, which in most cases obviously differ from each other far more
than do the varieties of the same species? How do those groups of
species, which constitute what are called distinct genera, and which
differ from each other more than do the species of the same genus,
arise? All these results, as we shall more fully see in the next
chapter, follow inevitably from the struggle for life. Owing to this
struggle for life, any variation, however slight and from whatever
cause proceeding, if it be in any degree profitable to an individual
of any species, in its infinitely complex relations to other organic
beings and to external nature, will tend to the preservation of that
individual, and will generally be inherited by its offspring. The
offspring, also, will thus have a better chance of surviving, for, of
the many individuals of any species which are periodically born, but a
small number can survive. I have called this principle, by which each
slight variation, if useful, is preserved, by the term of Natural
Selection, in order to mark its relation to man's power of selection.
We have seen that man by selection can certainly produce great
results, and can adapt organic beings to his own uses, through the
accumulation of slight but useful variations, given to him by the hand
of Nature. But Natural Selection, as we shall hereafter see, is a
power incessantly ready for action, and is as immeasurably superior to
man's feeble efforts, as the works of Nature are to those of Art.

We will now discuss in a little more detail the struggle for
existence. In my future work this subject shall be treated, as it well
deserves, at much greater length. The elder De Candolle and Lyell have
largely and philosophically shown that all organic beings are exposed
to severe competition. In regard to plants, no one has treated this
subject with more spirit and ability than W. Herbert, Dean of
Manchester, evidently the result of his great horticultural knowledge.
Nothing is easier than to admit in words the truth of the universal
struggle for life, or more difficult--at least I have found it
so--than constantly to bear this conclusion in mind. Yet unless it be
thoroughly engrained in the mind, I am convinced that the whole
economy of nature, with every fact on distribution, rarity, abundance,
extinction, and variation, will be dimly seen or quite misunderstood.
We behold the face of nature bright with gladness, we often see
superabundance of food; we do not see, or we forget, that the birds
which are idly singing round us mostly live on insects or seeds, and
are thus constantly destroying life; or we forget how largely these
songsters, or their eggs, or their nestlings, are destroyed by birds
and beasts of prey; we do not always bear in mind, that though food
may be now superabundant, it is not so at all seasons of each
recurring year.

I should premise that I use the term Struggle for Existence in a large
and metaphorical sense, including dependence of one being on another,
and including (which is more important) not only the life of the
individual, but success in leaving progeny. Two canine animals in a
time of dearth, may be truly said to struggle with each other which
shall get food and live. But a plant on the edge of a desert is said
to struggle for life against the drought, though more properly it
should be said to be dependent on the moisture. A plant which annually
produces a thousand seeds, of which on an average only one comes to
maturity, may be more truly said to struggle with the plants of the
same and other kinds which already clothe the ground. The missletoe is
dependent on the apple and a few other trees, but can only in a
far-fetched sense be said to struggle with these trees, for if too
many of these parasites grow on the same tree, it will languish and
die. But several seedling missletoes, growing close together on the
same branch, may more truly be said to struggle with each other. As
the missletoe is disseminated by birds, its existence depends on
birds; and it may metaphorically be said to struggle with other
fruit-bearing plants, in order to tempt birds to devour and thus
disseminate its seeds rather than those of other plants. In these
several senses, which pass into each other, I use for convenience sake
the general term of struggle for existence.

A struggle for existence inevitably follows from the high rate at
which all organic beings tend to increase. Every being, which during
its natural lifetime produces several eggs or seeds, must suffer
destruction during some period of its life, and during some season or
occasional year, otherwise, on the principle of geometrical increase,
its numbers would quickly become so inordinately great that no country
could support the product. Hence, as more individuals are produced
than can possibly survive, there must in every case be a struggle for
existence, either one individual with another of the same species, or
with the individuals of distinct species, or with the physical
conditions of life. It is the doctrine of Malthus applied with
manifold force to the whole animal and vegetable kingdoms; for in this
case there can be no artificial increase of food, and no prudential
restraint from marriage. Although some species may be now increasing,
more or less rapidly, in numbers, all cannot do so, for the world
would not hold them.

There is no exception to the rule that every organic being naturally
increases at so high a rate, that if not destroyed, the earth would
soon be covered by the progeny of a single pair. Even slow-breeding
man has doubled in twenty-five years, and at this rate, in a few
thousand years, there would literally not be standing room for his
progeny. Linnaeus has calculated that if an annual plant produced only
two seeds--and there is no plant so unproductive as this--and their
seedlings next year produced two, and so on, then in twenty years
there would be a million plants. The elephant is reckoned to be the
slowest breeder of all known animals, and I have taken some pains to
estimate its probable minimum rate of natural increase: it will be
under the mark to assume that it breeds when thirty years old, and
goes on breeding till ninety years old, bringing forth three pair of
young in this interval; if this be so, at the end of the fifth century
there would be alive fifteen million elephants, descended from the
first pair.

But we have better evidence on this subject than mere theoretical
calculations, namely, the numerous recorded cases of the astonishingly
rapid increase of various animals in a state of nature, when
circumstances have been favourable to them during two or three
following seasons. Still more striking is the evidence from our
domestic animals of many kinds which have run wild in several parts of
the world: if the statements of the rate of increase of slow-breeding
cattle and horses in South America, and latterly in Australia, had not
been well authenticated, they would have been quite incredible. So it
is with plants: cases could be given of introduced plants which have
become common throughout whole islands in a period of less than ten
years. Several of the plants now most numerous over the wide plains of
La Plata, clothing square leagues of surface almost to the exclusion
of all other plants, have been introduced from Europe; and there are
plants which now range in India, as I hear from Dr. Falconer, from
Cape Comorin to the Himalaya, which have been imported from America
since its discovery. In such cases, and endless instances could be
given, no one supposes that the fertility of these animals or plants
has been suddenly and temporarily increased in any sensible degree.
The obvious explanation is that the conditions of life have been very
favourable, and that there has consequently been less destruction of
the old and young, and that nearly all the young have been enabled to
breed. In such cases the geometrical ratio of increase, the result of
which never fails to be surprising, simply explains the
extraordinarily rapid increase and wide diffusion of naturalised
productions in their new homes.