especially as the plant's own anthers and pistil generally stand so close
together that self-fertilisation seems almost inevitable. Many flowers, on
the other hand, have their organs of fructification closely enclosed, as in
the great papilionaceous or pea-family; but in several, perhaps in all,
such flowers, there is a very curious adaptation between the structure of
the flower and the manner in which bees suck the nectar; for, in doing
this, they either push the flower's own pollen on the stigma, or bring
pollen from another flower. So necessary are the visits of bees to
papilionaceous flowers, that I have found, by experiments published
elsewhere, that their fertility is greatly diminished if these visits be
prevented. Now, it is scarcely possible that bees should fly from flower
to flower, and not carry pollen from one to the other, to the great good,
as I believe, of the plant. Bees will act like a camel-hair pencil, and it
is quite sufficient just to touch the anthers of one flower and then the
stigma of another with the same brush to ensure fertilisation; but it must
not be supposed that bees would thus produce a multitude of hybrids between
distinct species; for if you bring on the same brush a plant's own pollen
and pollen from another species, the former will have such a prepotent
effect, that it will invariably and completely destroy, as has been shown
by Gartner, any influence from the foreign pollen.
When the stamens of a flower suddenly spring towards the pistil, or slowly
move one after the other towards it, the contrivance seems adapted solely
to ensure self-fertilisation; and no doubt it is useful for this end: but,
the agency of insects is often required to cause the stamens to spring
forward, as Kolreuter has shown to be the case with the barberry; and
curiously in this very genus, which seems to have a special contrivance for
self-fertilisation, it is well known that if very closely-allied forms or
varieties are planted near each other, it is hardly possible to raise pure
seedlings, so largely do they naturally cross. In many other cases, far
from there being any aids for self-fertilisation, there are special
contrivances, as I could show from the writings of C. C. Sprengel and from
my own observations, which effectually prevent the stigma receiving pollen
from its own flower: for instance, in Lobelia fulgens, there is a really
beautiful and elaborate contrivance by which every one of the infinitely
numerous pollen-granules are swept out of the conjoined anthers of each
flower, before the stigma of that individual flower is ready to receive
them; and as this flower is never visited, at least in my garden, by
insects, it never sets a seed, though by placing pollen from one flower on
the stigma of another, I raised plenty of seedlings; and whilst another
species of Lobelia growing close by, which is visited by bees, seeds
freely. In very many other cases, though there be no special mechanical
contrivance to prevent the stigma of a flower receiving its own pollen,
yet, as C. C. Sprengel has shown, and as I can confirm, either the anthers
burst before the stigma is ready for fertilisation, or the stigma is ready
before the pollen of that flower is ready, so that these plants have in
fact separated sexes, and must habitually be crossed. How strange are
these facts! How strange that the pollen and stigmatic surface of the same
flower, though placed so close together, as if for the very purpose of
self-fertilisation, should in so many cases be mutually useless to each
other! How simply are these facts explained on the view of an occasional
cross with a distinct individual being advantageous or indispensable!
If several varieties of the cabbage, radish, onion, and of some other
plants, be allowed to seed near each other, a large majority, as I have
found, of the seedlings thus raised will turn out mongrels: for instance,
I raised 233 seedling cabbages from some plants of different varieties
growing near each other, and of these only 78 were true to their kind, and
some even of these were not perfectly true. Yet the pistil of each
cabbage-flower is surrounded not only by its own six stamens, but by those
of the many other flowers on the same plant. How, then, comes it that such
a vast number of the seedlings are mongrelized? I suspect that it must
arise from the pollen of a distinct variety having a prepotent effect over
a flower's own pollen; and that this is part of the general law of good
being derived from the intercrossing of distinct individuals of the same
species. When distinct species are crossed the case is directly the
reverse, for a plant's own pollen is always prepotent over foreign pollen;
In the case of a gigantic tree covered with innumerable flowers, it may be
objected that pollen could seldom be carried from tree to tree, and at most
only from flower to flower on the same tree, and that flowers on the same
tree can be considered as distinct individuals only in a limited sense. I
believe this objection to be valid, but that nature has largely provided
against it by giving to trees a strong tendency to bear flowers with
separated sexes. When the sexes are separated, although the male and
female flowers may be produced on the same tree, we can see that pollen
must be regularly carried from flower to flower; and this will give a
better chance of pollen being occasionally carried from tree to tree. That
trees belonging to all Orders have their sexes more often separated than
other plants, I find to be the case in this country; and at my request Dr.
Hooker tabulated the trees of New Zealand, and Dr. Asa Gray those of the
United States, and the result was as I anticipated. On the other hand, Dr.
Hooker has recently informed me that he finds that the rule does not hold
in Australia; and I have made these few remarks on the sexes of trees
simply to call attention to the subject.
Turning for a very brief space to animals: on the land there are some
hermaphrodites, as land-mollusca and earth-worms; but these all pair. As
yet I have not found a single case of a terrestrial animal which fertilises
itself. We can understand this remarkable fact, which offers so strong a
contrast with terrestrial plants, on the view of an occasional cross being
indispensable, by considering the medium in which terrestrial animals live,
and the nature of the fertilising element; for we know of no means,
analogous to the action of insects and of the wind in the case of plants,
by which an occasional cross could be effected with terrestrial animals
without the concurrence of two individuals. Of aquatic animals, there are
many self-fertilising hermaphrodites; but here currents in the water offer
an obvious means for an occasional cross. And, as in the case of flowers,
I have as yet failed, after consultation with one of the highest
authorities, namely, Professor Huxley, to discover a single case of an
hermaphrodite animal with the organs of reproduction so perfectly enclosed
within the body, that access from without and the occas
ional influence of a
distinct individual can be shown to be physically impossible. Cirripedes
long appeared to me to present a case of very great difficulty under this
point of view; but I have been enabled, by a fortunate chance, elsewhere to
prove that two individuals, though both are self-fertilising
hermaphrodites, do sometimes cross.
It must have struck most naturalists as a strange anomaly that, in the case
of both animals and plants, species of the same family and even of the same
genus, though agreeing closely with each other in almost their whole
organisation, yet are not rarely, some of them hermaphrodites, and some of
them unisexual. But if, in fact, all hermaphrodites do occasionally
intercross with other individuals, the difference between hermaphrodites
and unisexual species, as far as function is concerned, becomes very small.
From these several considerations and from the many special facts which I
have collected, but which I am not here able to give, I am strongly
inclined to suspect that, both in the vegetable and animal kingdoms, an
occasional intercross with a distinct individual is a law of nature. I am
well aware that there are, on this view, many cases of difficulty, some of
which I am trying to investigate. Finally then, we may conclude that in
many organic beings, a cross between two individuals is an obvious
necessity for each birth; in many others it occurs perhaps only at long
intervals; but in none, as I suspect, can self-fertilisation go on for
perpetuity.
Circumstances favourable to Natural Selection. -- This is an extremely
intricate subject. A large amount of inheritable and diversified
variability is favourable, but I believe mere individual differences
suffice for the work. A large number of individuals, by giving a better
chance for the appearance within any given period of profitable variations,
will compensate for a lesser amount of variability in each individual, and
is, I believe, an extremely important element of success. Though nature
grants vast periods of time for the work of natural selection, she does not
be said, 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 soon be exterminated.
In man's methodical selection, a breeder selects for some definite object,
and free intercrossing will wholly stop his work. But when many men,
without intending to alter the breed, have a nearly common standard of
perfection, and all try to get and breed from the best animals, much
improvement and modification surely but slowly follow from this unconscious
process of selection, notwithstanding a large amount of crossing with
inferior animals. Thus it will be in nature; for within a confined area,
with some place in its polity not so perfectly occupied as might be,
natural selection will always tend to preserve all the individuals varying
in the right direction, though in different degrees, so as better to fill
up the unoccupied place. But if the area be large, its several districts
will almost certainly present different conditions of life; and then if
natural selection be modifying and improving a species in the several
districts, there will be intercrossing with the other individuals of the
same species on the confines of each. And in this case the effects of
intercrossing can hardly be counterbalanced by natural selection always
tending to modify all the individuals in each district in exactly the same
manner to the conditions of each; for in a continuous area, the conditions
will generally graduate away insensibly from one district to another. The
intercrossing will most affect those animals which unite for each birth,
which wander much, and which do not breed at a very quick rate. Hence in
animals of this nature, for instance in birds, varieties will generally be
confined to separated countries; and this I believe to be the case. In
hermaphrodite organisms which cross only occasionally, and likewise in
animals which unite for each birth, but which wander little and which can
increase at a very rapid rate, a new and improved variety might be quickly
formed on any one spot, and might there maintain itself in a body, so that
whatever intercrossing took place would be chiefly between the individuals
of the same new variety. A local variety when once thus formed might
subsequently slowly spread to other districts. On the above principle,
nurserymen always prefer getting seed from a large body of plants of the
same variety, as the chance of intercrossing with other varieties is thus
lessened.
Even in the case of slow-breeding animals, which unite for each birth, we
must not overrate the effects of intercrosses in retarding natural
selection; for I can bring a considerable catalogue of facts, showing that
within the same area, varieties of the same animal can long remain
distinct, from haunting different stations, from breeding at slightly
different seasons, or from varieties of the same kind preferring to pair
together.
Intercrossing plays a very important part in nature in keeping the
individuals 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 I have already attempted to show
that we have reason to believe that occasional intercrosses take place with
all animals and with all plants. Even if these take place only at long
intervals, I am convinced that the young thus produced will gain so much in
vigour and fertility over the offspring from long-continued
self-fertilisation, that they will have a better chance of surviving and
propagating their kind; and thus, in the long run, the influence of
intercrosses, even at rare intervals, will be great. If there exist
organic beings which never intercross, uniformity of character can be
retained amongst them, as long as their conditions of life remain the same,
only through the principle of inheritance, and through natural selection
destroying any which depart from the proper type; but if their conditions
of life change and they undergo modification, uniformity of character can
be given to their modified offspring, solely by natural selection
preserving the same favourable variations.
Isolation, also, is an important element in the process of natural
selection. In a confined or isolated area, if not very large, the organic
and inorganic conditions of life will generally be in a great degree
uniform; so that natural selection will tend to modify all the individuals
of a varying species throughout the area in the same manner in relation to
the same conditions. Intercrosses, also, with the individuals of the same
species, which otherwise would have inhabited the surrounding and
differently circumstanced districts, will be prevented. But isolation
probably acts more efficiently in checking the immigration of better
adapted organisms, af
ter any physical change, such as of climate or
elevation of the land, &c.; and thus new places in the natural economy of
the country are left open for the old inhabitants to struggle for, and
become adapted to, through modifications in their structure and
constitution. Lastly, isolation, by checking immigration and consequently
competition, will give time for any new variety to be slowly improved; and
this may sometimes be of importance in the production of new species. 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 individuals supported on it will necessarily be very small;
and fewness of individuals will greatly retard the production of new
species through natural selection, by decreasing the chance of the
appearance of favourable variations.
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 total number
of the species inhabiting it, will be found to be 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. Hence an oceanic island at first sight seems to have been highly
favourable for the production of new species. But we may thus greatly
deceive ourselves, for to ascertain whether a small isolated area, or a
large open area like a continent, has been most favourable for the
production of new organic forms, we ought to make the comparison within
equal times; and this we are incapable of doing.
Although I do not doubt that isolation is of considerable importance in the
production of new species, on the whole I am inclined to believe that
largeness of area is of more importance, more especially in the production
of species, which will prove capable of enduring for a long period, and of
spreading widely. Throughout a great and open area, not only will there be
a better chance of favourable variations arising from the large number of
individuals of the same species there supported, but the conditions of life
are infinitely 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
competition with many others. Hence more new places will be formed, and
the competition to fill them will be more severe, on a large than on a
small and isolated area. Moreover, great areas, though now continuous,
owing to oscillations of level, will often have recently 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 probably have been in some respects highly favourable for
the production 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, will give rise to most new varieties and species, and will thus
play an important part in the changing history of the organic world.
We can, perhaps, on these views, understand some facts which will be again
alluded to in our chapter on geographical distribution; for instance, that
the productions of the smaller continent of Australia have formerly
yielded, and apparently are now yielding, before those of the larger
Europaeo-Asiatic area. Thus, also, it is that continental productions have
everywhere become so largely naturalised on islands. On a small island,