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Charles Darwin
English Scientist
1809-1882 A selection from ON THE ORIGIN OF SPECIES
Narrated by Alan Sklar
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Illustrations of the Action of Natural Selection
In order to make it clear how, as I believe, natural selection acts, I
must beg permission to give one or two imaginary illustrations. Let us
take the case of a wolf, which preys on various animals, securing some
by craft, some by strength, and some by fleetness; and let us suppose
that the fleetest prey, a deer for instance, had from any change in
the country increased in numbers, or that other prey had decreased in
numbers, during that season of the year when the wolf is hardest
pressed for food. I can under such circumstances see no reason to
doubt that the swiftest and slimmest wolves would have the best chance
of surviving, and so be preserved or selected,—provided always that
they retained strength to master their prey at this or at some other
period of the year, when they might be compelled to prey on other
animals. I can see no more reason to doubt this, than that man can
improve the fleetness of his greyhounds by careful and methodical
selection, or by that unconscious selection which results from each
man trying to keep the best dogs without any thought of modifying the
breed.
Even without any change in the proportional numbers of the animals on
which our wolf preyed, a cub might be born with an innate tendency to
pursue certain kinds of prey. Nor can this be thought very improbable;
for we often observe great differences in the natural tendencies of
our domestic animals; one cat, for instance, taking to catch rats,
another mice; one cat, bringing home winged
game, another hares or rabbits, and another hunting on marshy ground
and almost nightly catching woodcocks or snipes. The tendency to catch
rats rather than mice is known to be inherited. Now, if any slight
innate change of habit or of structure benefited an individual wolf,
it would have the best chance of surviving and of leaving offspring.
Some of its young would probably inherit the same habits or structure,
and by the repetition of this process, a new variety might be formed
which would either supplant or coexist with the parent-form of wolf.
Or, again, the wolves inhabiting a mountainous district, and those
frequenting the lowlands, would naturally be forced to hunt different
prey; and from the continued preservation of the individuals best
fitted for the two sites, two varieties might slowly be formed. These
varieties would cross and blend where they met; but to this subject of
intercrossing we shall soon have to return. I may add, that there are two
varieties of the wolf inhabiting the
Catskill 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.
Let us now take a more complex case. Certain plants excrete a sweet
juice, apparently for the sake of eliminating something injurious from
their sap: this is effected by glands at the base of the stipules in
some Leguminosae, and at the back of the leaf of the common laurel.
This juice, though small in quantity, is greedily sought by insects.
Let us now suppose a little sweet juice or nectar to be excreted by
the inner bases of the petals of a flower. In this case insects in
seeking the nectar would get dusted with pollen, and would certainly
often transport the pollen from one flower to the stigma of another
flower. The flowers of two distinct individuals of the same species
would thus get crossed; and the act of crossing, we have good reason
to believe (as will hereafter be more fully alluded to), would produce
very vigorous seedlings, which consequently would have the best chance
of flourishing and surviving. Some of these seedlings would probably
inherit the nectar-excreting power. Those individual flowers which had
the largest glands or nectaries, and which excreted most nectar, would
be oftenest visited by insects, and would be oftenest crossed; and so
in the long-run would gain the upper hand. Those flowers, also, which
had their stamens and pistils placed, in relation to the size and
habits of the particular insects which visited them, so as to favour
in any degree the transportal of their pollen from flower to flower,
would likewise be favoured or selected. 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 object of
fertilisation, its destruction appears 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; and those
individuals which produced more and more pollen, and had larger and
larger anthers, would be selected.
When our plant, by this process of the continued preservation or
natural selection of more and more attractive flowers, had been
rendered highly attractive to insects, they would, unintentionally on
their part, regularly carry pollen from flower to flower; and that
they can most effectually do this, I could easily show by many
striking instances. I will give only one—not as a very striking case,
but as likewise illustrating one step in the separation of the sexes
of plants, presently to be alluded to. Some holly-trees bear only male
flowers, which have four stamens producing rather a 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 microscope, and on all, without exception, there were
pollen-grains, and on some a profusion of pollen. 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 favourable to bees, nevertheless every female flower
which I examined had been effectually fertilised by the bees,
accidentally dusted with pollen, having flown from tree to tree in
search of nectar. But to return to our imaginary case: as soon as the
plant had been rendered so highly attractive to insects that pollen
was regularly carried from flower to flower, another process might
commence. No naturalist doubts the advantage of what has been called
the "physiological division of labour;" 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 labour, individuals with this tendency
more and more increased, would be continually favoured or selected,
until at last a complete separation of the sexes would be effected.
Let us now turn to the nectar-feeding insects in our imaginary case:
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 they can, with a very little more trouble,
enter by the mouth. Bearing such facts in mind, I can see no reason to
doubt that an accidental deviation in the size and form of the body,
or in the curvature and length of the proboscis, etc., far too slight
to be appreciated by us, might profit a bee or other insect, so that
an individual so characterised would be able to obtain its food more
quickly, and so have a better chance of living and leaving
descendants. Its descendants would probably inherit a tendency to a
similar slight deviation of structure. The tubes of the corollas of
the common red and 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. Thus it might be a
great advantage to the hive-bee to have a slightly longer or
differently constructed proboscis. On the other hand, I have found by
experiment that the fertility of clover greatly depends on bees
visiting and moving parts of the corolla, so as to push the pollen on
to the stigmatic surface. Hence, again, if humble-bees were to become
rare in any country, it might be a great advantage to the red clover
to have a shorter or more deeply divided tube to its corolla, so that
the hive-bee could visit its flowers. Thus I can understand how a
flower and a bee might slowly become, either simultaneously or one
after the other, modified and adapted in the most perfect manner to
each other, by the continued preservation of individuals presenting
mutual and slightly favourable deviations of structure.
I am well aware that this doctrine of natural selection, exemplified
in the above imaginary instances, is open to the same objections which
were at first urged against Sir Charles Lyell's noble views on "the
modern changes of the earth, as illustrative of geology;" but we now
very seldom hear the action, for instance, of the coast-waves, called
a trifling and insignificant cause, when applied to the excavation of
gigantic valleys or to the formation of the longest lines of inland
cliffs. Natural selection can act only by the preservation and
accumulation of infinitesimally small inherited 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, if it be a true principle,
banish the belief of the continued creation of new organic beings, or
of any great and sudden modification in their structure. More information about Charles Darwin from Wikipedia
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