Natural Selection

The principle of selection is so potent in the hands of man.

Does it apply in nature? How will the struggle for existence act in regard to variation?

Our domestic productions have a lot more variety than wild organisms.

How Strong is the Hereditary Tendency?

Under domestication, the whole organisation becomes plastic.

The mutual relations of all organic beings to each other and to their living conditions are infinitely complex and close-fitting.

In the course of thousands of generations, useful variations also occur in the wild.

The individuals which have any advantage over others, however slight, would have the best chance of surviving and of procreating.

Any injurious variation would be rigidly destroyed.

Natural Selection is this:

• preservation of favourable variations
• rejection of injurious variations

Variations neither useful nor injurious would not be affected by natural selection.

They would be left a fluctuating element, as we see in the “polymorphic” species.

We best understand the probable course of natural selection by looking into physical changes in the country, such as climate change.

The proportional numbers of its inhabitants would almost immediately undergo a change. Some might become extinct.

Any change in population, independent of climage change itself, would most seriously affect many of the others.

If the country were open on its borders, new forms would immigrate.

This also would seriously disturb the relations of some of the natives.

This is proven by the influence of a single introduced tree or mammal.

If an area is open to immigration, these same places would be seized on by intruders.

The course of time would lead to slight modifications.

Natural selection would thus have free scope for the work of improvement.

A change in the conditions of life, by specially acting on the reproductive system, causes or increases variability.

It gives a better chance of profitable variations occurring.

Unless profitable variations do occur, natural selection can do nothing.

An extreme amount of variability is not necessary.

Man can produce great results by adding up individual differences. Nature could do this far more easily as she has longer time at her disposal.

Any great physical change, such as of climate or unusual isolation, is not necessary for natural selection.

This is because all are struggling together with nicely balanced forces. Extremely slight modifications in the structure or habits of one inhabitant would often give it an advantage over others.

Nature can act on every internal organ, on every shade of constitutional difference, on the whole machinery of life.

Man selects only for his own good;

Nature only for that of the being which she tends. Every selected character is fully exercised by her; and the being is placed under well-suited conditions of life.

Man keeps the natives of many climates in the same country.

In the wild, the slightest difference of structure or constitution may turn the balanced scale in the struggle for life.

This is why nature’s productions are far ’truer’ in character than man’s productions.

They are infinitely better adapted to the most complex conditions of life.

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

It rejects that which is bad, preserving and adding up all that is good.

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

We see nothing of these slow changes in progress, until the hand of time has marked the long lapse of ages, and then so imperfect is our view into long past geological ages, that we only see that the forms of life are now different from what they formerly were.

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 preserving 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.

and hawks are guided by eyesight to their prey,–so much so, that on parts of the Continent persons are warned not to keep white pigeons, as being the most liable to destruction.

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.

Nor ought we to think that the occasional destruction of an animal of any particular colour would produce little effect: we should remember how essential it is in a flock of white sheep to destroy every lamb with the faintest trace of black.

In plants the down on the fruit and the colour of the flesh are considered by botanists as characters of the most trifling importance: yet we hear from an excellent horticulturist, Downing, that in the United States smooth-skinned fruits suffer far more from a beetle, a curculio, than those with down; that purple plums suffer far more from a certain disease than yellow plums; whereas another disease attacks yellow-fleshed peaches far more than those with other coloured flesh.

If, with all the aids of art, these slight differences make a great difference in cultivating the several varieties, assuredly, in a state of nature, where the trees would have to struggle with other trees and with a host of enemies, such differences would effectually settle which variety, whether a smooth or downy, a yellow or purple fleshed fruit, should succeed.

In looking at many small points of difference between species, which, as far as our ignorance permits us to judge, seem to be quite unimportant, we must not forget that climate, food, &c., probably produce some slight and direct effect. It is, however, far more necessary to bear in mind that there are many unknown laws of correlation of growth, which, when one part of the organisation is modified through variation, and the modifications are accumulated by natural selection for the good of the being, will cause other modifications, often of the most unexpected nature.

As we see that those variations which under domestication appear at any particular period of life, tend to reappear in the offspring at the same period.

For instance, in the seeds of the many varieties of our culinary and agricultural plants; in the caterpillar and cocoon stages of the varieties of the silkworm; in the eggs of poultry, and in the colour of the down of their chickens; in the horns of our sheep and cattle when nearly adult;–so in a state of nature, natural selection will be enabled to act on and modify organic beings at any age, by the accumulation of profitable variations at that age, and by their inheritance at a corresponding age.

If it profits a plant to have its seeds more widely disseminated by the wind, I can see no greater difficulty in this being effected through natural selection, than in the cotton-planter increasing and improving by 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 no doubt affect, through the laws of correlation, the structure of the adult; and probably in the case of those insects which live only for a few hours, and which never feed, a large part of their structure is merely the correlated result of successive changes in the structure of their larvae.

So, 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 in the least degree injurious: for if they became so, they would cause the extinction of the species.

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.