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04 - Natural Selection 04-06 - On the generality of Intercross Between Individuals of the Same Species 60 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 Leguminosae and at the backs of the leaves of the common laurel.

sap
sap

Leguminosae
Leguminosae

Laurel Wreath
Laurel Wreath


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.

insect
insect

nectar
nectar

pollen
pollen


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 insects which visited them, so as to favour in any degree the transportal of the pollen, would likewise be favoured.

stamen
stamen

pistil
pistil


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 fertilisation, 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.
04 - Natural Selection 04-06 - On the generality of Intercross Between Individuals of the Same Species 50 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 organisation being similarly acted on- of which fact numerous instances could be given with our domestic productions.

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.

Guillemot
Guillemot

Faroe Islands
Faroe Islands


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 successful in its battle for life, it would slowly spread from a central district, competing with and conquering the unchanged individuals on the margins of an ever-increasing circle.
04 - Natural Selection 04-06 - On the generality of Intercross Between Individuals of the Same Species 10 Illustrations of the Action of Natural Selection:
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04 - Natural Selection 04-06 - On the generality of Intercross Between Individuals of the Same Species 20 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 was
hardest pressed for food.

wolf
wolf


Under such circumstances 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 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 improve 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.