the race for life will have been less severe, and there will have been less
modification and less extermination. Hence, perhaps, it comes that the
flora of Madeira, according to Oswald Heer, resembles the extinct tertiary
flora of Europe. All fresh-water basins, taken together, make a small area
compared with that of the sea or of the land; and, consequently, the
competition between fresh-water productions will have been less severe than
elsewhere; new forms will have been more slowly formed, and old forms more
slowly exterminated. And it is in fresh water that we find seven genera of
Ganoid fishes, remnants of a once preponderant order: and in fresh water
we find some of the most anomalous forms now known in the world, as the
Ornithorhynchus and Lepidosiren, which, like fossils, connect to a certain
extent orders now widely separated in the natural scale. These anomalous
forms may almost be called living fossils; they have endured to the present
day, from having inhabited a confined area, and from having thus been
exposed to less severe competition.
To sum up the circumstances favourable and unfavourable to natural
selection, as far as the extreme intricacy of the subject permits. I
conclude, looking to the future, that for terrestrial productions a large
continental area, which will probably undergo many oscillations of level,
and which consequently will exist for long periods in a broken condition,
will be the most favourable for the production of many new forms of life,
likely to endure long and to spread widely. For the area will first have
existed as a continent, and the inhabitants, at this period numerous in
individuals and kinds, will have been subjected to very severe competition.
When converted by subsidence into large separate islands, there will still
exist many individuals of the same species on each island: intercrossing
on the confines of the range of each species will thus be checked: after
physical changes of any kind, immigration will be prevented, so that new
places in the polity of each island will have to be filled up by
modifications of the old inhabitants; and time will be allowed for the
varieties in each to become well modified and perfected. When, by renewed
elevation, the islands shall be re-converted into a continental area, there
will again be severe competition: the most favoured or improved varieties
will be enabled to spread: there will be much extinction of the less
improved forms, and the relative proportional numbers of the various
inhabitants of the renewed continent will again be changed; and again there
will be a fair field for natural selection to improve still further the
inhabitants, and thus produce new species.
That natural selection will always act with extreme slowness, I fully
admit. Its action depends on there being places in the polity of nature,
which can be better occupied by some of the inhabitants of the country
undergoing modification of some kind. The existence of such places will
often depend on physical changes, which are generally very slow, and on the
immigration of better adapted forms having been checked. But the action of
natural selection will probably still oftener depend on some of the
inhabitants becoming slowly modified; the mutual relations of many of the
other inhabitants being thus disturbed. Nothing can be effected, unless
favourable variations occur, and variation itself is apparently always a
very slow process. The process will often be greatly retarded by free
intercrossing. Many will exclaim that these several causes are amply
sufficient wholly to stop the action of natural selection. I do not
believe so. On the other hand, I do believe that natural selection will
always act very slowly, often only at long intervals of time, and generally
on only a very few of the inhabitants of the same region at the same time.
I further believe, that this very slow, intermittent action of natural
selection accords perfectly well with what geology tells us of the rate and
manner at which the inhabitants of this world have changed.
Slow though the process of selection may be, if feeble man can do much by
his powers of artificial selection, I can see no limit to the amount of
change, to the beauty and infinite complexity of the coadaptations between
all organic beings, one with another and with their physical conditions of
life, which may be effected in the long course of time by nature's power of
selection.
Extinction. -- This subject will be more fully discussed in our chapter on
Geology; but it must be here alluded to from being intimately connected
with natural selection. Natural selection acts solely through the
preservation of variations in some way advantageous, which consequently
endure. But as from the high geometrical powers of increase of all organic
beings, each area is already fully stocked with inhabitants, it follows
that as each selected and favoured form increases in number, so will the
less favoured forms decrease and become rare. Rarity, as geology tells us,
is the precursor to extinction. We can, also, see that any form
represented by few individuals will, during fluctuations in the seasons or
in the number of its enemies, run a good chance of utter extinction. But
we may go further than this; for as new forms are continually and slowly
being produced, unless we believe that the number of specific forms goes on
perpetually and almost indefinitely increasing, numbers inevitably must
become extinct. That the number of specific forms has not indefinitely
increased, geology shows us plainly; and indeed we can see reason why they
should not have thus increased, for the number of places in the polity of
nature is not indefinitely great,--not that we have any means of knowing
that any one region has as yet got its maximum of species. Probably no
region is as yet fully stocked, for at the Cape of Good Hope, where more
species of plants are crowded together than in any other quarter of the
world, some foreign plants have become naturalised, without causing, as far
as we know, the extinction of any natives.
Furthermore, the species which are most numerous in individuals will have
the best chance of producing within any given period favourable variations.
We have evidence of this, in the facts given in the second chapter, showing
that it is the common species which afford the greatest number of recorded
varieties, or incipient species. Hence, rare species will be less quickly
modified or improved within any given period, and they will consequently be
beaten in the race for life by the modified descendants of the commoner
species.
From these several considerations I think it inevitably follows, that as
new species in the course of time are formed through natural selection,
others will become rarer and rarer, and finally extinct. The forms which
stand in closest competition with those undergoing modification and
improvement, will naturally suffer most. And we have seen in the chapter
on the Struggle for Existence that it is the most closely-allied
forms,--varieties of th
e same species, and species of the same genus or of
related genera,--which, from having nearly the same structure,
constitution, and habits, generally come into the severest competition with
each other. Consequently, each new variety or species, during the progress
of its formation, will generally press hardest on its nearest kindred, and
tend to exterminate them. We see the same process of extermination amongst
our domesticated productions, through the selection of improved forms by
man. Many curious instances could be given showing how quickly new breeds
of cattle, sheep, and other animals, and varieties of flowers, take the
place of older and inferior kinds. In Yorkshire, it is historically known
that the ancient black cattle were displaced by the long-horns, and that
these 'were swept away by the short-horns' (I quote the words of an
agricultural writer) 'as if by some murderous pestilence.'
Divergence of Character. -- The principle, which I have designated by this
term, is of high importance on my theory, and explains, as I believe,
several important facts. In the first place, varieties, even
strongly-marked ones, though having somewhat of the character of
species--as is shown by the hopeless doubts in many cases how to rank
them--yet certainly differ from each other far less than do good and
distinct species. Nevertheless, according to my view, varieties are
species in the process of formation, or are, as I have called them,
incipient species. How, then, does the lesser difference between varieties
become augmented into the greater difference between species? That this
does habitually happen, we must infer from most of the innumerable species
throughout nature presenting well-marked differences; whereas varieties,
the supposed prototypes and parents of future well-marked species, present
slight and ill-defined differences. Mere chance, as we may call it, might
cause one variety to differ in some character from its parents, and the
offspring of this variety again to differ from its parent in the very same
character and in a greater degree; but this alone would never account for
so habitual and large an amount of difference as that between varieties of
the same species and species of the same genus.
As has always been my practice, let us seek light on this head from our
domestic productions. We shall here find something analogous. A fancier
is struck by a pigeon having a slightly shorter beak; another fancier is
struck by a pigeon having a rather longer beak; and on the acknowledged
principle that 'fanciers do not and will not admire a medium standard, but
like extremes,' they both go on (as has actually occurred with
tumbler-pigeons) choosing and breeding from birds with longer and longer
beaks, or with shorter and shorter beaks. Again, we may suppose that at an
early period one man preferred swifter horses; another stronger and more
bulky horses. The early differences would be very slight; in the course of
time, from the continued selection of swifter horses by some breeders, and
of stronger ones by others, the differences would become greater, and would
be noted as forming two sub-breeds; finally, after the lapse of centuries,
the sub-breeds would become converted into two well-established and
distinct breeds. As the differences slowly become greater, the inferior
animals with intermediate characters, being neither very swift nor very
strong, will have been neglected, and will have tended to disappear. Here,
then, we see in man's productions the action of what may be called the
principle of divergence, causing differences, at first barely appreciable,
steadily to increase, and the breeds to diverge in character both from each
other and from their common parent.
But how, it may be asked, can any analogous principle apply in nature? I
believe it can and does apply most efficiently, from the simple
circumstance that the more diversified the descendants from any one species
become in structure, constitution, and habits, by so much will they be
better enabled to seize on many and widely diversified places in the polity
of nature, and so be enabled to increase in numbers.
We can clearly see this in the case of animals with simple habits. Take
the case of a carnivorous quadruped, of which the number that can be
supported in any country has long ago arrived at its full average. If its
natural powers of increase be allowed to act, it can succeed in increasing
(the country not undergoing any change in its conditions) only by its
varying descendants seizing on places at present occupied by other animals:
some of them, for instance, being enabled to feed on new kinds of prey,
either dead or alive; some inhabiting new stations, climbing trees,
frequenting water, and some perhaps becoming less carnivorous. The more
diversified in habits and structure the descendants of our carnivorous
animal became, the more places they would be enabled to occupy. What
applies to one animal will apply throughout all time to all animals--that
is, if they vary--for otherwise natural selection can do nothing. So it
will be with plants. It has been experimentally proved, that if a plot of
ground be sown with several distinct genera of grasses, a greater number of
plants and a greater weight of dry herbage can thus be raised. The same
has been found to hold good when first one variety and then several mixed
varieties of wheat have been sown on equal spaces of ground. Hence, if any
one species of grass were to go on varying, and those varieties were
continually selected which differed from each other in at all the same
manner as distinct species and genera of grasses differ from each other, a
greater number of individual plants of this species of grass, including its
modified descendants, would succeed in living on the same piece of ground.
And we well know that each species and each variety of grass is annually
sowing almost countless seeds; and thus, as it may be said, is striving its
utmost to increase its numbers. Consequently, I cannot doubt that in the
course of many thousands of generations, the most distinct varieties of any
one species of grass would always have the best chance of succeeding and of
increasing in numbers, and thus of supplanting the less distinct varieties;
and varieties, when rendered very distinct from each other, take the rank
of species.
The truth of the principle, that the greatest amount of life can be
supported by great diversification of structure, is seen under many natural
circumstances. In an extremely small area, especially if freely open to
immigration, and where the contest between individual and individual must
be severe, we always find great diversity in its inhabitants. For
instance, I found that a piece of turf, three feet by four in size, which
had been exposed for many years to exactly the same conditions, supported
twenty species of plants, and these belonged to eighteen genera and to
eight orders, which shows how much these plants differed from each other.
So it is with the plants and insects on small and uniform islets; and so in
r /> small ponds of fresh water. Farmers find that they can raise most food by
a rotation of plants belonging to the most different orders: nature
follows what may be called a simultaneous rotation. Most of the animals
and plants which live close round any small piece of ground, could live on
it (supposing it not to be in any way peculiar in its nature), and may be
said to be striving to the utmost to live there; but, it is seen, that
where they come into the closest competition with each other, the
advantages of diversification of structure, with the accompanying
differences of habit and constitution, determine that the inhabitants,
which thus jostle each other most closely, shall, as a general rule, belong
to what we call different genera and orders.
The same principle is seen in the naturalisation of plants through man's
agency in foreign lands. It might have been expected that the plants which
have succeeded in becoming naturalised in any land would generally have
been closely allied to the indigenes; for these are commonly looked at as
specially created and adapted for their own country. It might, also,
perhaps have been expected that naturalised plants would have belonged to a
few groups more especially adapted to certain stations in their new homes.
But the case is very different; and Alph. De Candolle has well remarked in
his great and admirable work, that floras gain by naturalisation,
proportionally with the number of the native genera and species, far more
in new genera than in new species. To give a single instance: in the last
edition of Dr. Asa Gray's 'Manual of the Flora of the Northern United
States,' 260 naturalised plants are enumerated, and these belong to 162
genera. We thus see that these naturalised plants are of a highly
diversified nature. They differ, moreover, to a large extent from the
indigenes, for out of the 162 genera, no less than 100 genera are not there
indigenous, and thus a large proportional addition is made to the genera of
these States.
By considering the nature of the plants or animals which have struggled
successfully with the indigenes of any country, and have there become
naturalised, we can gain some crude idea in what manner some of the natives
would have had to be modified, in order to have gained an advantage over
the other natives; and we may, I think, at least safely infer that
diversification of structure, amounting to new generic differences, would
have been profitable to them.
The advantage of diversification in the inhabitants of the same region is,
in fact, the same as that of the physiological division of labour in the
organs of the same individual body--a subject so well elucidated by Milne
Edwards. No physiologist doubts that a stomach by being adapted to digest
vegetable matter alone, or flesh alone, draws most nutriment from these
substances. So in the general economy of any land, the more widely and
perfectly the animals and plants are diversified for different habits of
life, so will a greater number of individuals be capable of there
supporting themselves. A set of animals, with their organisation but
little diversified, could hardly compete with a set more perfectly
diversified in structure. It may be doubted, for instance, whether the
Australian marsupials, which are divided into groups differing but little
from each other, and feebly representing, as Mr. Waterhouse and others have
remarked, our carnivorous, ruminant, and rodent mammals, could successfully
compete with these well-pronounced orders. In the Australian mammals, we
see the process of diversification in an early and incomplete stage of
development.
After the foregoing discussion, which ought to have been much amplified, we
may, I think, assume that the modified descendants of any one species will
succeed by so much the better as they become more diversified in structure,