Natural Selection Recap

Domestication produces a lot of variability.

This is mainly due to the reproductive system being susceptible to changes in the conditions of life.

• This makes the reproductive system fail to reproduce offspring exactly like the parent-form.

Variability is governed by many complex laws:

• correlation of growth
• use and disuse
• the direct action of the physical conditions of life.

As long as the conditions of life remain the same, a modification which has already been inherited for many generations, may continue to be inherited for an almost infinite number of generations.

On the other hand, variability, when it has once come into play, does not wholly cease.

New varieties are still occasionally produced by our most anciently domesticated productions.

Man does not actually produce variability.

• He only unintentionally exposes organic beings to new conditions of life
• Then nature acts on the organisation, and causes variability.

But man selects the variations given to him by nature, and thus accumulate them in any desired manner.

• He thus adapts animals and plants for his own benefit or pleasure.

He may do this:

• methodically, or
• unconsciously by preserving the individuals most useful to him at the time, without any thought of altering the breed.

He can influence the character of a breed by selecting, in each successive generation, individual differences.

Many of the breeds produced by man have the character of natural species.

The principles which have acted so efficiently under domestication should also act under nature.

We see the most powerful and ever-acting means of selection in the preservation of favoured individuals and races during the Struggle for Existence.

The struggle for existence inevitably follows from the high geometrical ratio of increase which is common to all organic beings.

This high rate of increase is proved by calculation from:

• the effects of a succession of peculiar seasons, and
• the results of naturalisation

More individuals are born than can possibly survive.

A grain in the balance will determine:

• which individual shall live and which shall die
• which variety or species shall increase in number and which shall decrease or become extinct.

The individuals of the same species come into the closest competition with each other in all respects.

• The struggle will generally be most severe between them.

It will be almost equally severe between the varieties of the same species,

• It is next in severity between the species of the same genus.

But the struggle will often be very severe between beings most remote in the scale of nature.

The balance is turned by the slightest advantage that one being has over its competition.

With animals having separated sexes there will in most cases be a struggle between the males for possession of the females.

Those that will leave most progeny are those who:

• are most vigorous
• have most successfully struggled with their conditions of life

Success will often depend on:

• having special weapons or means of defence, or
• the charms of the males

The slightest advantage will lead to victory.

In geology, each land has undergone great physical changes.

We might have expected that organic beings would have varied in the wild just as in domestication.

It has often been asserted that the amount of variation in the wild is limited.

• But this is quite incapable of proof

By adding up mere individual differences in his domestic productions, man can quickly produce a great result even if he capriciously acts on external characters alone.

There are at least individual differences in wild species.

But, besides such differences, all naturalists have admitted the existence of noteworthy varieties.

There is no clear distinction:

• between:
  • individual differences and
  • slight varieties
• between
  • more plainly marked varieties and sub-species, and
  • species.

If we have variability in the wild and a powerful agent always ready to act and select, then useful variations would be preserved, accumulated, and inherited.

Man has patience to select variations most useful to himself.

Nature should also select useful variations for her living products.

I can see no limit to this power, in slowly and beautifully adapting each form to the most complex relations of life.

The theory of natural selection is in itself probable.