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ganization, are hermaphrodites, and some unisexual. But if, in fact, all
hermaphrodites do occasionally intercross, the difl'erence between them and
unisexual species is, as far as function is concerned, very small.

From these several considerations and from the many special facts which
I have collected, but which I am unable here to give, it appears that with
animals and plants an occasional intercross between distinct individuals is
a very general, if not universal, law of nature.



This is an extremely intricate subject. A great amount of variability,
under which term individual differences are always included, will evidently
be favorable. A large number of individuals, by giving a better chance
within any given period for the appearance of profitable variations, will
compensate for a lesser amount of variability in each individual, and is, I
believe, a highly important element of success. Though nature grants long
periods of time for the work of natural selection, she does not grant an
indefinite period, for as all organic beings are striving to seize on each
place in the economy of nature, if any one species does not become
modified and improved in a corresponding degree with its competitors it
will be exterminated. Unless favorable variations be inherited by some at
least of the offspring, nothing can be effected by natural selection. The
tendency to reversion may often check or prevent the work; but as this
tendency has not prevented man from forming by selection numerous
domestic races, why should it prevail against natural selection?

In the case of methodical selection, a breeder selects for some definite
object, and if the individuals be allowed freely to intercross, his work will
completely fail. But when many men, without intending to alter the breed,
have a nearly common standard of perfection, and all try to procure and
breed from the best animals, improvement surely but slowly follows from
this unconscious process of selection, notwithstanding that there is no separa-
tion of selected individuals. Thus it will be under nature; for within a
•confined area, with some place in the natural poUty not perfectly occupied,
all the individuals varying in the right direction, though in different de-
grees, will tend to be preserved. But if the area be large, its several districts
will almost certainly present different conditions of life; and then, if the
same species undergoes modification in different districts, the newly formed
Varieties will intercross on the confines of each. But we shall see in the
sixth chapter that intermediate varieties, inhabiting intermediate districts,
will in the long-run generally be supplanted by one of the adjoining
Varieties. Intercrossing will chiefly affect those animals which unite for
each birth and wander much, and which do not breed at a very quick
rate. Hence with animals of this nature, for instance birds, varieties will
generally be confined to separated countries; and this I find to be the case.
With hermaphrodite organisms which cross only occasionally, and likewise
for animals which unite for each birth, but which wander little and can
increase at a rapid rate, a new and improved variety might be quickly
formed on any one spot, and might there maintain itself in a body and
afterward spread, so that the individuals of the new variety would chiefly
cross together. On this principle nurserymen always prefer saving seed
from a large body of plants, as the chance of intercrossing is tRus lessened.

Even with animals which unite for each birth, and which do not propa-


gate rapidly, we must not assume that free intercrossing would always
eliminate the efTects of natural selection; for I can bring forward a con-
siderable body of facts showing that within the same area two varieties of
the same animal may long remain distinct, from haunting different sta-
tions, from breeding at slightly different seasons, or from the individuals
of each variety preferring to pair together.

Intercrossing plays a very important part in nature by keeping the in-
dividuals of the same species, or of the same variety, true and uniform in
character. It will obviously thus act far more efficiently with those animals
which unite for each birth; but, as already stated, we have reason to be-
lieve that occasional intercrosses take place with all animals and plants.
Even if these take place only at long intervals of time, the young thus
produced will gain so much in vigor and fertility over the offspring from
long-continued self-fertiKzation, that they will have a better chance of sur-
viving and propagating their kind; and thus in the long-run the influence
of crosses, even at rare intervals, will be great. With respect to organic
beings extremely low in the scale, which do not propagate sexually, nor
conjugate, and which cannot possibly intercross, uniformity of character
can be retained by them under the same conditions of life, only through
the principle of inheritance, and through natural selection which will de-
stroy any individuals departing from the proper type. If the conditions of
life change, and the form undergoes modification, uniformity of character
can be given to the modified offspring, solely by natural selection preserv-
ing similar favorable variations.

Isolation also is an important element in the modification of species
through natural selection. In a confined or isolated area, if not very large,
the organic and inorganic conditions of life will generally be almost uni-
form; so that natural selection will tend to modify all the varying indi-
viduals of the same species in the same manner. Intercrossing with the
inhabitants of the surrounding districts will also be thus prevented. Moritz
Wagner has lately published an interesting essay on this subject, and has
shown that the service rendered by isolation in preventing crosses between
newly-formed varieties is probably greater even than I supposed. But from
reasons already assigned I can by no means agree with this naturalist, that
migration and isolation are necessary elements for the formation of new
species. The importance of isolation is Kkewise great in preventing, after
any physical change in the conditions, such as of climate, elevation of the *
land, etc., the immigration of better adapted organisms; and thus new
places in the natural economy of the district will be left open to be filled
up by the modification of the old inhabitants. Lastly, isolation will give
time for a new variety to be improved at a slow rate; and this may some-
times be of much importance. If, however, an isolated area be very small,
either from being surrounded by barriers, or from having very peculiar
physical conditions, the total number of the inhabitants will be small; and
this will retard the production of new species through natural selection,
by decreasing the chances of favorable variations arising.


The mere lapse of time by itself does nothing, either for or against
natural selection. I state this because it has been erroneously asserted that
the element of time has been assumed by me to play an all-important part
in modifying species, as if all the forms of life were necessarily undergoing
change through some innate law. Lapse of time is only so far important,
and its importance in this respect is great, that it gives a better chance of
beneficial variations arising and of their being selected, accumulated, and
; fixed. It likewise tends to increase the direct action of the physical condi-
I tions of life, in relation to the constitution of each organism.

If we turn to nature to test the truth of these remarks, and look at any
small isolated area, such as an oceanic island, although the number of
species inhabiting it is small, as we shall see in our chapter on Geographical
Distribution; yet of these species a very large proportion are endemic, —
that is, have been produced there and nowhere else in the world. Hence
an oceanic island at first sight seems to have been highly favorable for the
production of new species. But we may thus deceive ourselves, for to ascer-
tain whether a small isolated area, or a large open area like a continent,
has been most favorable for the production of new organic forms, we ought
to make the comparison within equal times; and this we are incapable
of doing.

Although isolation is of great importance in the production of new
species, on the whole I am inclined to believe that largeness of area is still
more important, especially for the production of species which shall prove
capable of enduring for a long period, and of spreading widely. Through-
out a great and open area, not only will there be a better chance of
favorable variations, arising from the large number of individuals of the
same species there supported, but the conditions of life are much more
complex from the large number of already existing species; and if some
of these many species become modified and improved, others will have to
be improved in a corresponding degree, or they will be exterminated. Each
new form, also, as soon as it has been much improved, will be able to
spread over the open and continuous area, and will thus come into com-
petition with many other forms. Moreover, great areas, though now con-
tinuous, will often, owing to former oscillations of level, have existed in a
broken condition; so that the good effects of isolation will generally, to a
certain extent, have concurred. Finally, I conclude that, although small
isolated areas have been in some respects highly favorable for the produc-
tion of new species, yet that the course of modification will generally have
been more rapid on large areas; and what is more important, that the
new forms produced on large areas, which already have been victorious
over many competitors, will be those that will spread most widely, and
will give rise to the greatest number of new varieties and species. They
will thus play a more important part in the changing history of the organic
^ world.

In accordance with this view, we can, perhaps, understand some facts
which will be again alluded to in our chapter on Geographical Distribu-


tion; for instance, the fact of the productions of the smaller continent of
Australia now yielding before those of the larger Europaso- Asiatic area.
Thus, also, it is that continental productions have everywhere become so
largely naturalized on islands. On a small island, the race for life will have
been less severe, and there will have been less modification and less ex-
termination. Hence, we can understand how it is that the flora of Madeira,
according to Oswald Heer, resembles to a certain extent 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. Consequently, the competi-
tion between fresh-water productions will have been less severe than else-
where, new forms will have been then more slowly produced, and old
forms more slowly exterminated. And it is in fresh-water basins that we
find seven genera of Ganoid fishes, remnants of a once preponderant
order: and in fresh water w^e 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 at present widely sundered in the
natural scale. These anomalous forms may be called living fossils; they
have endured to the present day, from having inhabited a confined area,
and from having been exposed to less varied, and therefore less severe,

To sum up, as far as the extreme intricacy of the subject permits, the
circumstances favorable and unfavorable for the production of new species
through natural selection. I conclude that for terrestrial productions a
large continental area, which has undergone many oscillations of level,
will have been the most favorable for the production of many new forms
of life, fitted to endure for a long time and to spread widely. While the
area existed as a continent, the inhabitants will have been numerous in
individuals and kinds, and will have been subjected to severe competition.
When converted by subsistence into large separate islands, there will still
have existed many individuals of the same species on each island: inter-
crossing on the confines of the range of each new species will have been
checked: after physical changes of any kind, immigration will have been
prevented, so that new places in the polity of each island will have had to
be filled up by the modification of the old inhabitants; and time will have
been allowed for the varieties in each to become well modified and per-
fected. When, by renewed elevation, the islands were reconverted into a
continental area, there will again have been very severe competition; the
most favored or improved varieties will have been enabled to spread;
there will have been much extinction of the less improved forms, and the
relative proportional numbers of the various inhabitants of the reunited
continent will again have been changed; and again there will have been a
fair field for natural selection to improve still further the inhabitants, and
thus to produce new species.

That natural selection generally acts with extreme slowness, I fully
admit. It can act only when there are places in the natural polity of a
district which can be better occupied by the modification of some of its


existing inhabitants. The occurrence of such places will often depend on
physical changes, which generally take place very slowly, and on the
immigration of better adapted forms being prevented. As some few of the
old inhabitants become modified, the mutual relations of others will often
be disturbed; and this will create new places, ready to be filled up by
better adapted forms; but all this will take place very slowly. Although all
the individuals of the same species differ in some slight degree from each
other, it would often be long before differences of the right nature in
various parts of the organization might occur. The result would often be
greatly retarded by free intercrossing. Many will exclaim that these several
causes are amply sufficient to neutralize the power of natural selection. I
do not believe so. But I do believe that natural selection will generally act
very slowly, only at long intervals of time, and only on a few of the in-
habitants of the same region. I further believe that these slow, intermittent
results accord well with what geology tells us of the rate and manner at
which the inhabitants of the world have changed.

Slow though the process of selection may be, if feeble man can do much
by artificial selection, I can see no limit to the amount of change, to the
beauty and complexity of the coadaptations between all organic beings,
one with another and with their physical conditions of life, which may
have been effected in the long course of time through nature's power of
selection, that is, by the survival of the fittest.


This subject will be more fully discussed in our chapter on Geology; but
it must here be alluded to from being intimately connected with natural
selection. Natural selection acts solely through the preservation of varia-
tions in some way advantageous, which consequently endure. Owing to
the high geometrical rate of increase of all organic beings, each area is
already fully stocked with inhabitants; and it follows from this, that as the
favored forms increase in number, so, generally, will the less-favored
decrease and become rare. Rarity, as geology tells us, is the precursor to
extinction. We can see that any form which is represented by few indi-
viduals will run a good chance of utter extinction, during great fluctua-
tions in the nature of the seasons, or from a temporary increase in the
number of its enemies. But we may go further than this; for, as new forms
are produced, unless we admit that specific forms can go on indefinitely
increasing in number, many old forms must become extinct. That the
number of specific forms has not indefinitely increased, geology plainly
tells us; and we shall presently attempt to show why it is that the number
of species throughout the world has not become immeasurably great.

We have seen that the species which are most numerous in individuals
have the best chance of producing favorable variations within any given
period. We have evidence of this, in the facts stated in the second chapter,
showing that it is the common and diffused or dominant species which


offer the greatest number of recorded varieties. Hence, rare species will be
less quickly modified or improved within any given period; they will con-
sequently be beaten in the race for life by the modified and improved
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 the same
species, and species of the same genus or 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 among 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 his-
torically 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."


The principle, which I have designated by this term, is of high importance,
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 far less from each other than do good and distinct spe-
cies. 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 a degree of difference as that
between the species of the same genus.

As has always been my practice, I have sought light on this head from our
domestic productions. We shall here find something analogous. It will be ad-
mitted that the production of races so different as short-horn and Hereford
cattle, race and cart horses, the several breeds of pigeons, etc., could never


have been effected by the mere chance accumulation of similar variations
during many successive generations. In practice, a fancier is, for instance,
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 ex-
tremes," they both go on (as has actually occurred with the sub-breeds of the
tumbler-pigeon) 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 of history, the men of one nation or district required swifter
horses, while those of another required stronger and bulkier horses. The early
differences would be very slight; but, in the course of time, from the con-
tinued selection of swifter horses in the one case, and of stronger ones in the
other, the differences would become greater, and would be noted as forming
two sub-breeds. Ultimately, after the lapse of centuries, these sub-breeds
would become converted into two well-established and distinct breeds. As
the differences became greater, the inferior animals with intermediate char-
acters, being neither very swift nor very strong, would not have been used for
breeding, and will thus 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

But, how, it may be asked, can any analogous principle apply in nature?
I believe it can and does apply most efficiently (though it was a long time
before I saw how), 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

We can clearly discern this in the case of animals with simple habits. Take
the case of a carnivorous quadruped, of which the number that can be sup-
ported in any country has long ago arrived at its full average. If its natural
power of increase be allowed to act, it can succeed in increasing (the country
not undergoing any change in conditions) only by its varying descendants
seizing on places at present occupied by other animals : some of them, for in-
stance, 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 animals become, the more places they will
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
effect nothing. So it will be with plants. It has been experimentally proved,
that if a plot of ground be sown with one species of grass, and a similar plot
be sown with several distinct genera of grasses, a greater number of plants
and a greater weight of dry herbage can be raised in the latter than the
former case. The same has been found to hold good when one variety and


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 the varieties
were continually selected which differed from each other in the same man-
ner, though in a very slight degree, as do the distinct species and genera of
grasses, a greater number of individual plants of this species, including its
modified descendants, would succeed in living on the same piece of ground.
And we know that each species and each variety of grass is annually sowing
almost countless seeds; and is thus striving, as it may be said, to the utmost
to increase in number. Consequently, in the course of many thousand gener-
ations, the most distinct varieties of any one species of grass would have the
best chance of succeeding and of increasing in numbers, and thus of supplant-
ing 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

Online LibraryCharles DarwinThe origin of species → online text (page 10 of 50)