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 



