Page - 114 - in The Origin of Species

114 ORIGIN OF SPECIES with each other in their whole organisation, are hermaphro- dites, and some unisexual. But if, in fact, all hermaphro- dites do occasionally intercross, the difference between them and unisexual species is, as far as function is concerned, very- small. From these several considerations and from the many special facts which I have collected, but which I am unable here to give, it appears that with animals and plants an oc- casional intercross between distinct individuals is a very gen- eral, if not universal, law of nature. 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 al- ways included, will evidently be favourable. A large num- ber of individuals, by giving a better chance within any given period for the appearance of profitable variations, will com- pensate for a lesser amount of variability in each individual, and is, I believe, a highly 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 vari- ations be inherited by some at least of the offspring, nothing can be effected by natural selection. The tendency to rever- sion may often check or prevent the work; but as this ten- dency has not prevented man from forming by selection nu- merous domestic races, why should it prevail against natural selection? In the case of methodical selection, a breeder selects for some definite object, and if the individuals be allowed freely to intercross, his work will completely fail. But when many men, without intending to alter the breed, have a nearly com- mon standard of perfection, and all try to procure and breed from the best animals, improvement surely but slowly follows from this unconscious process of selection, notwithstanding