Charles Darwin.

The origin of species online

. (page 9 of 50)
Online LibraryCharles DarwinThe origin of species → online text (page 9 of 50)
Font size
QR-code for this ebook

a deer for instance, had from any change in the country increased in num-
bers, or that other prey had decreased in numbers, during that season of
the year when the wolf was hardest pressed for food. Under such circum-
stances the swiftest and slimmest wolves have the best chance of surviving,
and so being preserved or selected, provided always that they retain
strength to master their prey at this or some other period of the year, when
they were compelled to prey on other animals. I can see no more reason to
doubt that this would be the result, than that man should be able to im-
prove the fleetness of his greyhounds by careful and methodical selection,
or by that kind of unconscious selection which follows from each man trying
to keep the best dogs without any thought of modifying the breed. I may
add that, according to Mr. Pierce, there are two varieties of the wolf in-
habiting the Gatskill Mountains, in the United States, one with a light
greyhound-like form, which pursues deer, and the other more bulky, with
shorter legs, which more frequently attacks the shepherd's flocks.

It should be observed that in the above illustration, I speak of the slim-
mest individual wolves, and not of any single strongly marked variation
having been preserved. In former editions of this work I sometimes spoke
as if this latter alternative had frequently occurred. I saw the great im-
portance of individual differences, and this led me fully to discuss the re-
sults of unconscious selection by man, which depends on the preservation
of all the more or less valuable individuals, and on the destruction of the
worst. I saw, also, that the preservation in a state of nature of any occa-
sional deviation of structure, such as a monstrosity, would be a rare event;
and that, if at first preserved, it would generally be lost by subsequent


intercrossing with ordinary individuals. Nevertheless, until reading an able
and valuable article in the North British Review (1867), I did not ap-
preciate how rarely single variations, whether slight or strongly marked,
could be perpetuated. The author takes the case of a pair of animals, pro-
ducing during their lifetime two hundred offspring, of which, from various
causes of destruction, only two on an average survive to procreate their
kind. This is rather an extreme estimate for most of the higher animals, but
by no means so for many of the lower organisms. He then shows that if a
single individual were born, which varied in some manner, giving it twice
as good a chance of life as that of the other individuals, yet the chances
would be strongly against its survival. Supposing it to survive and to breed,
and that half its young inherited the favorable variation; still, as the re-
viewer goes on to show, the young would have only a slightly better chance
of surviving and breeding; and this chance would go on decreasing in the
succeeding generations. The justice of these remarks cannot, I think, be
disputed. If, for instance, a bird of some kind could procure its food more
easily by having its beak curved, and if one were born with its beak strongly
curved, and which consequently flourished, nevertheless there would be a
very poor chance of this one individual perpetuating its kind to the exclu-
sion of the common form ; but there can hardly be a doubt, judging by what
we see taking place under domestication, that this result would follow from
the preservation during many generations of a large number of individuals
with more or less strongly curved beaks, and from the destruction of a still
larger number with the straightest beaks.

It should not, however, be overlooked, that certain rather strongly
marked variations, which no one would rank as mere individual differences,
frequently recur owing to a similar organization being similarly acted on —
of which fact numerous instances could be given with our domestic produc-
tions. In such cases, if the varying individual did not actually transmit to
its offspring its newly acquired character, it would undoubtedly transmit
to them, as long as the existing conditions remained the same, a still
stronger tendency to vary in the same manner. There can also be little
doubt that the tendency to vary in the same manner has often been so
strong that all the individuals of the same species have been similarly
modified without the aid of any form of selection. Or only a third, fifth, or
tenth part of the individuals may have been thus affected, of which fact
several instances could be given. Thus Graba estimates that about one-fifth
of the guillemots in the Faroe Islands consist of a variety so well marked,
that it was formerly ranked as a distinct species under the name of Uria
lacrymans. In cases of this kind, if the variation were of a beneficial nature,
the original form would soon be supplanted by the modified form, through
the survival of the fittest.

To the effects of intercrossing in eliminating variations of all kinds, I
shall have to recur: but it may be here remarked that most animals and
plants keep to their proper homes, and do not needlessly wander about; we
see this even with migratory birds, which almost always return to the same


spot. Consequently each newly-formed variety would generally be at first
local, as seems to be the common rule with varieties in a state of nature;
so that similarly modified individuals would soon exist in a small body
together, and would often breed together. If the new variety were success-
ful in its battle for life, it would slowly spread from a central district, com-
peting with and conquering the unchanged individuals on the margins of
an ever-increasing circle.

It may be worth while to give another and more complex illustration of
the action of natural selection. Certain plants excrete sweet juice, apparently
for the sake of eliminating something injurious from the sap: this is
effected, for instance, by glands at the base of the stipules in some Legu-
minosae, and at the backs of the leaves of the common laurel. This juice,
though small in quantity, is greedily sought by insects; but their visits do
not in any way benefit the plant. Now, let us suppose that the juice or
nectar was excreted from the inside of the flowers of a certain number of
plants of any species. Insects in seeking the nectar would get dusted with
pollen, and would often transport it from one flower to another. The
flowers of two distinct individuals of the same species would thus get
crossed ; and the act of crossing, as can be fully proved, gives rise to vigorous
seedlings, which consequently would have the best chance of flourishing
and surviving. The plants which produced flowers with the largest glands
or nectaries, excreting most nectar, would oftenest be visited by insects, and
would oftenest be crossed; and so in the long-run would gain the upper
hand and form a local variety. The flowers also, which had their stamens
and pistils placed, in relation to the size and habits of the particular insect
which visited them, so as to favor in any degree the transportal of the.
pollen, would likewise be favored. We might have taken the case of insects
visiting flowers for the sake of collecting pollen instead of nectar; and as
pollen is formed for the sole purpose of fertilization, its destruction appears
to be a simple loss to the plant; yet if a little pollen were carried, at first
occasionally and then habitually, by the pollen-devouring insects from
flower to flower, and a cross thus effected, although nine-tenths of the
pollen were destroyed it might still be a great gain to the plant to be thus
robbed; and the individuals which produced more and more pollen, and
had larger anthers, would be selected.

When our plant, by the above process long continued, had been rendered
highly attractive to insects, they would, unintentionally on their part,
regularly carry pollen from flower to flower: and that they do this effectu-
ally I could easily show by many striking facts. I will give only one, as
likewise illustrating one step in the separation of the sexes of plants. Some
holly-trees bear only male flowers, which have four stamens producing a
rather small quantity of pollen, and a rudimentary pistil; other holly- trees
bear only female flowers; these have a full-sized pistil, and four stamens
with shrivelled anthers, in which not a grain of pollen can be detected.
Having found a female tree exactly sixty yards from a male tree, I put the
stigmas of twenty flowers, taken from different branches, under the micro-


scope, and on all, without exception, there were a few pollen-grains, and
on some a profusion. As the wind had set for several days from the female
to the male tree, the pollen could not thus have been carried. The weather
had been cold and boisterous and therefore not favorable to bees, neverthe-
less every female flower which I examined had been effectually fertilized
by the bees, which had flown from tree to tree in search of nectar. But to
return to our imaginary case; as soon as the plant had been render.ed so
highly attractive to insects that pollen was regularly carried from flower
to flower, another process might commence. No naturalist doubts the ad-
vantage of what has been called the "physiolofflcal diyisioii_of labor" ; hence
we may believe that it would be advantageous to a plant to produce stamens
alone in one flower or on one whole plant, and pistils alone in another
flower or on another plant. In plants under culture and placed under new
conditions of life, sometimes the male organs and sometimes the female
organs become more or less impotent; now if we suppose this to occur in
ever so slight a degree under nature, then, as pollen is already carried
regularly from flower to flower, and as a more complete separation of the
sexes of our plant would be advantageous on the principle of the division
of labor, individuals with this tendency more and more increased would be
continually favored or selected, until at last a complete separation of the
sexes might be effected. It would take up too much space to show the
various steps, through dimorphism and other means, by which the separa-
tion of the sexes in plants of various kinds is apparently now in progress;
but I may add that some of the species of holly in North America are, ac-
cording to Asa Gray, in an exactly intermediate condition, or, as he ex-
presses it, are more less dioeciously polygamous.

Let us now turn to the nectar-feeding insects; we may suppose the plant,
of which we have been slowly increasing the nectar by continued selection,
to be a common plant; and that certain insects depended in main part on
its nectar for food. I could give many facts showing how anxious bees are
to save time: for instance, their habit of cutting holes and sucking the
nectar at the bases of certain flowers, which with a very little more trouble
they can enter by the mouth. Bearing such facts in mind, it may be believed
that under certain circumstances individual differences in the curvature or
length of the proboscis, etc., too slight to be appreciated by us, might profit
a bee or other insect, so that certain individuals would be able to obtain
their food more quickly than others; and thus the communities to which
they belonged would flourish and throw off many swarms inheriting the
same peculiarities. The tubes of the corolla of the common red or incarnate
clovers (Trifolium pratense and incarnatum) do not on a hasty glance
appear to differ in length; yet the hive-bee can easily suck the nectar out of
the incarnate clover, but not out of the common red clover, which is
visited by humble-bees alone, so that whole fields of the red clover offer in
vain an abundant supply of precious nectar to the hive-bee. That this nectar
is much liked by the hive-bee is certain; for I have repeatedly seen, but only
in the autumn, many hive-bees sucking the flowers through holes bitten in


the base of the tube by humble-bees. The difference in the length of the
corolla in the two kinds of clover, which determines the visits of the hive-
bee, must be very trifling; for I have been assured that when red clover has
been mown, the flowers of the second crop are somewhat smaller, and that
these are visited by many hive-bees. I do not know whether this statement
is accurate; nor whether another published statement can be trusted,
namely, that the Ligurian bee, which is generally considered a mere variety
of the common hive-bee, and which freely crosses with it, is able to reach
and suck the nectar of the red clover. Thus, in a country where this kind
of clover abounded, it might be a great advantage to the hive-bee to have
a slightly longer or differently constructed proboscis. On the other hand,
as the fertility of this clover absolutely depends on bees visiting the flowers,
if humble-bees were to become rare in any country, it might be a great
advantage to the plant to have a shorter or more deeply divided corolla, so
that the hive-bees should be enabled to suck its flowers. Thus I can under-
stand how a flower and a bee might slowly become, either simultaneously
or one after the other, modified and adapted to each other in the most
perfect manner, by the continued preservation of all the individuals which
presented slight deviations of structure mutually favorable to each other.

I am well aware that this doctrine of natural selection, exemplified in the
above imaginary instances, is open to the same objections which were first
urged against Sir Charles Lyell's noble views on "the modern changes of
the earth, as illustrative of geology"; but we now seldom hear the agencies
which we see still at work, spoken of as trifling or insignificant, when used
in explaining the excavation of the deepest valleys or the formation of
long lines of inland cliffs. Natural selection acts only by the preservation
and accumulation of smalMsiSFi^^^ modifications, each profitable to the
preserved being; and as modern geology has almost banished such views
as the excavation of a great valley by a single diluvial wave, so will natural
selection banish the belief of the continued creation of new organic beings,
or of any great and sudden modification in their structure.


I must here introduce a short digression. In the case of animals and
plants with separated sexes, it is of course obvious that two individuals
must always (with the exception of the curious and not well understood
cases of parthenogenesis) unite for each birth; but in the case of hermaph-
rodites this is far from obvious. Nevertheless there is reason to believe
that with all hermaphrodites two individuals, either occasionally or habitu-
ally, concur for the reproduction of their kind. This view was long ago
doubtfully suggested by Sprengel, Knight, and Kolreuter. We shall pres-
ently see its importance; but I must here treat the subject with extreme
brevity, though I have the materials prepared for an ample discussion. All
vertebrate animals, all insects, and some other large groups of animals,
pair for each birth. Modem research has much diminished the number of


supposed hermaphrodites, and of real hermaphrodites a large number
pair; that is, two individuals regularly unite for reproduction, which is all
that concerns us. But still there are many hermaphrodite animals which
certainly do not habitually pair, and a vast majority of plants are her-
maphrodites. What reason, it may be asked, is there for supposing in
these cases that two individuals ever concur in reproduction? As it is im-
possible here to enter on details, I must trust to some general considera-
tions alone.

In the first place, I have collected so large a body of facts, and made so
many experiments, showing, in accordance with the almost universal be-
lief of breeders, that with animals and plants a cross between different
varieties, or between individuals of the same variety but of another strain,
gives vigor and fertility to the offspring; and on the other hand, that close
interbreeding diminishes vigor and fertility; that these facts alone incline
me to believe that it is a general law of nature that no organic being
fertilizes itself for a perpetuity of generations; but that a cross with another
individual is occasionally — perhaps at long intervals of time — indispensable.

On the belief that this is a law of nature, we can, I think, understand
several large classes of facts, such as the following, which on any other
view are inexplicable. Every hybridizer knows how unfavorable exposure
to wet is to the fertilization of a flower, yet what a multitude of flowers
have their anthers and stigmas fully exposed to the weather! If an occa-
sional cross be indispensable, notwithstanding that the plant's own anthers
and pistil stand so near each other as almost to insure self-fertilization, the
fullest freedom for the entrance of pollen from another individual will
explain the above state of exposure of the organs. Many flowers, on the
other hand, have their organs of fructification closely enclosed, as in the
great papilionaceous or pea-family; but these almost invariably present
beautiful and curious adaptations in relation to the visits of insects. So
necessary are the visits of bees to many papilionaceous flowers, that their
fertility is greatly diminished if these visits be prevented. Now, it is scarcely
possible for insects to fly from flower to flower, and not to carry pollen
from one to the other, to the great good of the plant. Insects act like a
camel-hair pencil, and it is sufficient, to insure fertilization, just to touch
with the same brush the anthers of one flower and then the stigma of
another; but it must not be supposed that bees would thus produce a
multitude of hybrids between distinct species; for if a plant's own pollen
and that from another species are placed on the same stigma, the former
is so prepotent that it invariably and completely destroys, as has been
shown by Gartner, the influence of the foreign pollen.

When the stamens of a flower suddenly spring toward the pistil, or slowly
move one after the other toward it, the contrivance seems adapted solely
to insure self-fertilization; 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 in this very
genus, which seems to have a special contrivance for self-fertilization, it is


well known that, if closely-allied forms or varieties are planted near each
other, it is hardly possible to raise pure seedlings, so largely do they natu-
rally cross. In numerous other cases, far from self-fertilization being
favored, there are special contrivances which effectually prevent the stigma
receiving pollen from its own flower, as I could show from the works of
Sprengel and others, as well as from my own observations : for instance, in
Lobelia fulgens, there is a really beautiful and elaborate contrivance by
which all the infinitely numerous pollen-granules are swept out of the con-
joined 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 raise plenty of seedlings. Another species
of Lobelia, which is visited by bees, seeds freely in my garden. In very
many other cases, though there is no special mechanical contrivance to
prevent the stigma receiving pollen from the same flower, yet, as Sprengel
and more recently Hildebrand and others have shown, and as I can con-
firm, either the anthers burst before the stigma is ready for fertilization, or
the stigma is ready before the pollen of that flower is ready, so that these
so-named dichogamous plants have in fact separated sexes, and must habit-
uaUy be crossed. So it is with the reciprocally dimorphic and trimorphic
plants previously alluded to. 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-fertilization, should be in so
many cases mutually useless to each other! How simply are these facts ex-
plained 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 of the seed-
lings thus raised turn out, as I found, 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 sur-
rounded not only by its own six stamens, but by those of the many other
flowers on the same plant; and the pollen of each flower readily gets on
its stigma without insect agency; for I have found that plants carefully
protected from insects produce the full number of pods. How, then, comes
it that such a vast number of the seedlings are mongrelized? It must arise
from the pollen of a distinct variety having a prepotent effect over the
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 reversed, for a plant's own
pollen is almost always prepotent over foreign pollen; but to this subject
we shall return in a future chapter.

In the case of a large 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 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, pollen must be regularly carried from
flower to flower; and this will give a better chance of pollen being occa-
sionally 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 informs me that the rule
does not hold good in Australia; but if most of the Australian trees are
dichogamous, the same result would follow as if they bore flowers with
separated sexes. I have made these few remarks on trees simply to call at-
tention to the subject.

Turning for a brief space to animals: various terrestrial species are
hermaphrodites, such as the land-mollusca and earth-worms; but these all
pair. As yet I have not found a single terrestrial animal which can fertilize
itself. This"~Temafkable fact, which off'ers so strong a contrast with terrestrial
plants, is intelligible on the view of an occasional cross being indispensable;
for owing to the nature of the fertilizing element there are no means,
analogous to the action of insects and of the wind with plants, by which
an occasional cross could be eflfected with terrestrial animals without the
concurrence of two individuals. Of aquatic animals, there are many self-
fertilizing hermaphrodites; but here the currents of water offer an obvious
means for an occasional cross. As in the case of flowers, I have as yet
failed, after consultation with one of the highest authorities, viz.. Professor
Huxley, to discover a single hermaphrodite animal with the organs of re-
production so perfectly enclosed that access from without, and the occa-
sional influence of a distinct individual, can be shown to be physically
impossible. Cirripedes long appeared to me to present, under this point of
view, a case of great difficulty; but I have been enabled, by a fortunate
chance, to prove that two individuals, though both of self-fertilizing her-
maphrodites, do sometimes cross.

It must have struck most naturalists as a strange anomaly that, both
with animals and plants, some species of the same family and even of the
same genus, though agreeing closely with each other in their whole or-

Online LibraryCharles DarwinThe origin of species → online text (page 9 of 50)