Origin of Species Variorum

occasional intercross with a distinct individual is a law of nature. I am well aware that there are, on this view, many cases of difficulty, some of which I am trying to 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 for the production of new forms through Natural Selection.

This is an extremely intricate subject. A great amount of variability, under which term individual differences are always included, will evidently be favourable. A large number of individuals, by giving a better chance for the appearance of profitable variations within any given period, will compensate for a lesser amount of variability in each individual, and is, I believe, an extremely important element of success. Though Nature grants long 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 .. be exterminated. Unless favourable variations be inherited by some at least of the offspring, nothing can be effected by natural selection. ↑ The tendency to reversion may often check or prevent the work; but as this tendency has not prevented man from forming by selection numerous domestic races, why should it prevail against natural selection?

In the case of methodical selection, a breeder selects for some definite object, and free intercrossing will wholly stop