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 III
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.
In a state of nature almost every plant produces seed, and amongst animals
there are very few which do not annually pair. Hence we may confidently
assert, that all plants and animals are tending to increase at a
geometrical ratio, that all would most rapidly stock every station in which
they could any how exist, and that the geometrical tendency to increase
must be checked by destruction at some period of life. Our familiarity
with the larger domestic animals tends, I think, to mislead us: we see no
great destruction falling on them, and we forget that thousands are
annually slaughtered for food, and that in a state of nature an equal
number would have somehow to be disposed of.
The only difference between organisms which annually produce eggs or seeds
by the thousand, and those which produce extremely few, is, that the
slow-breeders would require a few more years to people, under favourable
conditions, a whole district, let it be ever so large. The condor lays a
couple of eggs and the ostrich a score, and yet in the same country the
condor may be the more numerous of the two: the Fulmar petrel lays but one
egg, yet it is believed to be the most numerous bird in the world. One fly
deposits hundreds of eggs, and another, like the hippobosca, a single one;
but this difference does not determine how many individuals of the two
species can be supported in a district. A large number of eggs is of some
importance to those species, which depend on a rapidly fluctuating amount
of food, for it allows them rapidly to increase in number. But the real
importance of a large number of eggs or seeds is to make up for much
destruction at some period of life; and this period in the great majority
of cases is an early one. If an animal can in any way protect its own eggs
or young, a small number may be produced, and yet the average stock be
fully kept up; but if many eggs or young are destroyed, many must be
produced, or the species will become extinct. It would suffice to keep up
the full number of a tree, which lived on an average for a thousand years,
if a single seed were produced once in a thousand years, supposing that
this seed were never destroyed, and could be ensured to germinate in a
fitting place. So that in all cases, the average number of any animal or
plant depends only indirectly on the number of its eggs or seeds.
In looking at Nature, it is most necessary to keep the foregoing
considerations always in mind--never to forget that every single organic
being around us may be said to be striving to the utmost to increase in
numbers; that each lives by a struggle at some period of its life; that
heavy destruction inevitably falls either on the young or old, during each
generation or at recurrent intervals. Lighten any check, mitigate the
destruction ever so little, and the number of the species will almost
instantaneously increase to any amount. The face of Nature may be compared
to a yielding surface, with ten thousand sharp wedges packed close together
and driven inwards by incessant blows, sometimes one wedge being struck,
and then another with greater force.
What checks the natural tendency of each species to increase in number is
most obscure. Look at the most vigorous species; by as much as it swarms
in numbers, by so much will its tendency to increase be still further
increased. We know not exactly what the checks are in even one single
instance. Nor will this surprise any one who reflects how ignorant we are
on this head, even in regard to mankind, so incomparably better known than
any other animal. This subject has been ably treated by several authors,