Origin of Species Variorum

investigate. Finally then, we may conclude that in many organic beings, a cross between two individuals is an obvious necessity for each birth; in many others it occurs perhaps only at long intervals; but in none, as I suspect, can self-fertilisation go on for perpetuity.

Circumstances favourable to Natural Selection. —

This is an extremely intricate subject. A large amount of variability will evidently be favourable for the work of natural selection, but mere individual differences probably suffice. A large number of individuals, by giving a better chance for the appearance within any given period of profitable variations, will compensate for a lesser amount of variability in each individual, and is, I believe, an extremely important element of success. Though Nature grants vast periods of time for the work of natural selection, she does not grant an indefinite period; for as all organic beings are striving ... to seize on each place in the economy of nature, if any one species does not become modified and improved in a corresponding degree with its competitors, it will soon be exterminated. Unless favourable variations be inherited by some at least of the offspring, nothing can be effected by natural selection. Non-inheritance of any new character is, in fact, the same thing as reversion to the character of the grandparents or more remote ancestors; and no doubt the tendency to reversion may often have checked or prevented the action of natural selection; but its importance has been greatly exaggerated by some writers. If the tendency to reversion has not prevented man from creating innumerable hereditary races in the animal and vegetable kingdoms, why should it have stopped the process of natural selection?

In man's methodical selection, a breeder selects for