Natural Selection

Natural selection is scrutinising daily and hourly, throughout the world, every variation, even the slightest.

It:

• rejects the bad
• preserves and adds up the good

It silently and insensibly works, whenever and wherever opportunity offers, improving each organic being in relation to its organic and inorganic conditions of life.

We do not see these slow changes until time has long elapsed.

Natural selection can act only through and for the good of each being.

Yet trifling characters and structures may thus be acted on.

The colors of birds and insects preserve them from danger.

If grouse are not destroyed at some period of their lives, they would increase in countless numbers.

They suffer largely from birds of prey.

Natural selection might be most effective in:

• giving the proper colour to each kind of grouse
• keeping that colour, when once acquired, true and constant.

But it also does the occasional destruction of animals.

In a flock of white sheep, it is essential to destroy every lamb with the faintest trace of black.

In plants, botanists consdier the following as trifling:

• the down on the fruit
• the colour of the flesh

Yet the excellent horticulturist Downing says that in the US:

• smooth-skinned fruits suffer far more from a beetle than those with down
• purple plums suffer more from a certain disease than yellow plums
• yellow-fleshed peaches suffer from another disease more than those with other coloured flesh.

These slight differences make a great difference in cultivating the several varieties.

In the wild state, trees would have to struggle with:

• other trees and
• enemies

Such differences would settle which variety should succeed.

Climate, food, &c., probably produce some slight and direct effects on the tiny differences between species.

But we should remember that there are many unknown laws of correlation of growth.

One part is modified and accumulated by natural selection will cause other unexpected modifications.

For example, those variations which under domestication appear at a period of life tend to reappear in the offspring at the same period.

This is seen in:

• the seeds of plants
• the caterpillar and cocoon stages of the silkworm
• the eggs of poultry
• the colour of the feather of their chickens
• in the horns of our sheep and cattle when nearly adult

In a wild state, natural selection modifies organic beings at any age by:

• the accumulation of profitable variations at that age
• their inheritance at a corresponding age.

Natural selection is why a plant spreads its seeds by the wind just as the cotton-planter increases by artificial selection the down in the pods on his cotton-trees.

Natural selection may modify and adapt the larva of an insect to a score of contingencies, wholly different from those which concern the mature insect.

These modifications will affect, through the laws of correlation, the structure of the adult.

Conversely, modifications in the adult will probably often affect the structure of the larva.

But in all cases, natural selection will ensure that modifications consequent on other modifications at a different period of life, shall not be injurious.

If they became so, they would make the species extinct.

Natural selection will modify the structure of the young in relation to the parent, and of the parent in relation to the young.

In social animals it will adapt the structure of each individual for the benefit of the community; if each in consequence profits by the selected change.

What natural selection cannot do, is to modify the structure of one species, without giving it any advantage, for the good of another species; and though statements to this effect may be found in works of natural history, I cannot find one case which will bear investigation.

A structure used only once in an animal’s whole life, if of high importance to it, might be modified to any extent by natural selection; for instance, the great jaws possessed by certain insects, and used exclusively for opening the cocoon–or the hard tip to the beak of nestling birds, used for breaking the egg.

Of the best short-beaked tumbler-pigeons more perish in the egg than are able to get out of it; so that fanciers assist in the act of hatching.

If nature had to make the beak of a full-grown pigeon very short for the bird’s own advantage, the process of modification would be very slow, and there would be simultaneously the most rigorous selection of the young birds within the egg, which had the most powerful and hardest beaks, for all with weak beaks would inevitably perish: or, more delicate and more easily broken shells might be selected, the thickness of the shell being known to vary like every other structure.