Charles Darwin.

The origin of species online

. (page 6 of 50)
Online LibraryCharles DarwinThe origin of species → online text (page 6 of 50)
Font size
QR-code for this ebook


show, that, in any limited country, the species which are the 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), oftenest 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 spe-
cies — those which range widely, 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 de-
gree permanent, necessarily have to struggle with the other inhabitants of the
country, the species which are already dominant will be the more likely to
yield offspring, which, though in some slight degree modified, still inherit
those advantages that enabled their parents to become dominant over their
compatriots. In these remarks on predominance, it should be understood that
reference is made only to the forms which come into competition with each
other, and more especially to the members of the same genus or class having
nearly similar habits of life. With respect to the number of individuals, or
commonness of species, the comparison of course relates only to the members
of the same group. One of the higher plants may be said to be dominant if it
be more numerous in individuals and more widely diffused than the other
plants of the same country, which live under nearly the same conditions. A
plant of this kind is not the less dominant because some conferva inhabiting
the water or some parasitic fungus is infinitely more numerous in individuals,
and more widely diffused. But if the conferva or parasitic fungus exceeds its

allies in the above respects, it will then be dominant within its own class.

I

SPECIES OF THE LARGER GENERA IN EACH COUNTRY VARY MORE FREQUENTLY
THAN THE SPECIES OF THE SMALLER GENERA

If the plants inhabiting a country, as described in any Flora, be divided
into two equal masses, all those in the larger genera {i.e., those including
many species) being placed on one side, and all those in the smaller genera
on the other side, the former will be found to include a somewhat larger num-



36 THE ORIGIN OF SPECIES

ber of the very common and much diffused or dominant species. This 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 in-
organic conditions of that country favorable to the genus; and, consequently,
we might have expected to have found in the larger genera, or those includ-
ing many species, a larger 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 generally
have very wide ranges and are much diffused, but this seems to be connected
with the nature of the stations inhabited by them, and has Httle or no rela-
tion to the size of the genera to which the species belong. Again, plants low in
the scale of organization 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 organized 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, circum-
stances have been favorable for variation; and hence we might expect that
the circumstances would generally still be favorable 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 presented
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 least genera, with
from only one to four species, are altogether 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 manu-
factory 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 holds true
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



VARIATION UNDER NATURE 37

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 certainly holds good.

MANY OF THE SPECIE3 INCLUDED W^ITHIN THE LARGER GENERA RESEMBLE
VARIETIES IN BEING VERY CLOSELY, BUT UNEQUALLY, RELATED TO EACH
OTHER, AND IN HAVIITG RESTRICTED RANGES

There are other relations between the species of large genera and their
recorded varieties which deserve notice. We have seen that there is no infal-
lible criterion by which to distinguish species and well-marked varieties ; and
when intermediate links have not been found between doubtful forms, natu-
ralists 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 West-
wood in regard to insects, that in large genera the amount of difference be-
tween the species is often exceedingly small. I have endeavored to test this
numerically by averages, and, as far as my imperfect results go, they confirm
the view. I have also consulted some sagacious and 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 less than the usual amount of difference.

Moreover, the species of the larger 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 gener-
ally clustered like satellites around 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 di-



38 THE ORIGIN OF SPECIES

vergence of character, we shall see how this may be explained, and how the
lesser differences between varieties tend to increase into the greater differ-
ences between species.

There is one other point which is 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 would be reversed. But there is reason
to believe that the species which are very closely allied to other species, and
in so far resemble varieties, often have much restricted ranges. For instance,
Mr. H. G. Watson has marked for me in the well-sifted London Catalogue
of Plants (4th edition) sixty- three plants which are therein ranked as spe-
cies, but which he considers as so closely allied to other species as to
be of doubtful value: these sixty-three reputed species range on an aver-
age over 6.9 of the provinces into which Mr. Watson has divided Great
Britain. Now, in this same catalogue, fifty-three 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 nearly the same restricted average range, as have the 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.

SUMMARY

Finally, varieties cannot be distinguished from species, — except, first, by
the discovery of intermediate linking forms; and, secondly, by a certain in-
definite amount of difference between them; for two forms, if differing very
little, are generally ranked as varieties, notwithstanding that they cannot be
closely connected ; but the amount of difference considered necessary to give
to any two forms the rank of species cannot be defined. 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 other species. Species very closely alUed to other species apparently
have restricted ranges. In all these respects the species of large genera present
a stropg analogy with varieties. And we can clearly understand these anal-
ogies, if species once existed as varieties, and thus originated; whereas, these
analogies are utterly inexplicable if species are independent creations.

V/e have also seen that it is the most flourishing or dominant species of the
larger genera within each class which on an average yield the greatest num-
ber of varieties; and varieties, as we shall hereafter see, tend to become con-
verted into new and distinct species. Thus the larger genera 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

Its Bearing on Natural Selection — ^The Term used in a Wide Sense — Geometrical
Ratio of Increase — Rapid Increase of Naturalized Animals and Plants — Nature
of the Checks to Increase — Competition Universal — Effects of Climate — Protec-
tion 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 prelim-
inary remarks to show how the struggle for existence bears on natural selec-
tion. It has been seen in the last chapter that among 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 en-
titled 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 organization to another part, and to the con-
ditions of life, and of one organic being to another being, been perfected?
We see these beautiful coadaptations most plainly in the woodpecker and
the mistletoe; 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 in-
cipient 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 consti-
tute 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 from the struggle for life. Owing
to this struggle, variations, however slight and from whatever cause proceed-
ing, if they be in any degree profitable to the individuals of a species, in their
infinitely complex relations to other organic beings and to their physical con-
ditions of life, will tend to the preservation of such individuals, and will gen-
erally be inherited by the 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
natural selection, in order to mark its relation to man's power of selection.

39



40 THE ORIGIN OF SPECIES

But the expression often used by Mr. Herbert Spencer, of the Survival of the
Fittest, is more accurate, and is sometimes equally convenient. We have seen
that man by selection can certainly produce great results, and can adapt or-
ganic beings to his own uses, through the accumulation of slight but useful
variations, given to him by the hand of Nature. But Natural Selection, we
shall hereafter see, is a power incessantly ready for action, and is as immeas-
urably 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 will be treated, as it well deserves, at 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 knowl-
edge. Nothing is easier than to admit in words the truth of the universal
struggle for life, or more difficult — at least I found it so — than constantly to
bear. this conclusion in mind. Yet unless it be thoroughly ingrained in the
mind, the whole economy of nature, with every fact on distribution, rarity,
abundance, extinction, and variation, will be dimly seen or quite misunder-
stood. We behold the face of nature bright with gladness, we often see super-
abundance 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 de-
stroying 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 sea-
sons of each recurring year.

THE TERM, STRUGGLE FOR EXISTENCE, USED IN A LARGE SENSE

I should premise that I use this term in a large and metaphorical sense, in-
cluding 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 only one of an average 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 mistletoe 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
languishes and dies. But several seedling mistletoes, growing close together
on the same branch, may more truly be said to struggle with each other. As
the mistletoe is disseminated by birds, its existence depends on them; and it
may metaphorically be said to struggle with other fruit-bearing plants, in
tempting the birds to devour and thus disseminate its seeds. In these several



STRUGGLE FOR EXISTENCE

senses, which pass into each other, I use for convenience' sake the general
term of Struggle for Existence.

GEOMETRICAL RATIO OF INCREASE

A struggle for existence inevitably follows from the high rate at which all
organic beings tend to increase. Every being, which during its natural Hfe-
time 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 condi-
tions 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 arti-
ficial 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 in-
creases at so high a rate, that, if not destroyed, the earth would soon be cov-
ered by the progeny of a single pair. Even slow-breeding man has doubled in
twenty-five years, and at this rate, in less than a 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 unpro-
ductive 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
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 safest to
assume that it begins breeding when thirty years old, and goes on breeding
till ninety years old, bringing forth six young in the interval, and surviving
till one hundred years old; if this be so, after a period of from 740 to 750
years there would be nearly nineteen million elephants alive descended from
the first pair.

But we have better evidence on this subject than mere theoretical calcula-
tions, namely, the numerous recorded cases of the astonishingly rapid increase
of various animals in a state of nature, when circumstances have been favor-
able 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 incredible. So it is with
plants; cases could be given of introduced plants which have become com-
mon throughout whole islands in a period of less than ten years. Several of
the plants, such as the cardoon and a tall thistle, which are now the com-
monest over the wide plains of La Plata, clothing square leagues of surface



42 THE ORIGIN OF SPECIES

almost to the exclusion of every other plant, 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 Himalayas, which have been imported
from America since its discovery. In such cases, and endless others could be
given, no one supposes that the fertility of the animals or plants has been
suddenly and temporarily increased in any sensible degree. The obvious ex-
planation is that the conditions of life have been highly favorable, and that
there has consequently been less destruction of the old and young, and that
nearly all the young have been enabled to breed. Their geometrical ratio of
increase, the result of which never fails to be surprising, simply explains their
extraordinarily rapid increase and wide diffusion in their new homes.

In a state of nature almost every full-grown plant annually produces seed,
and among 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 rapidly stock every station in which
they could anyhow exist — and that this 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 de-
struction falling on them, but we do not keep in mind that thousands are
annually slaughtered for food, and that in a state of nature an equal num-
ber 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 favorable con-
ditions, 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 fluctuating amount of food, for it allows
them rapidly to increase in numbers. But the real importance of a large num-



Online LibraryCharles DarwinThe origin of species → online text (page 6 of 50)