148 THE ORIGIN OF SPECIES 



that two individuals, though both are self-fertilizing 

 hermaphrodites, do sometimes cross. 



It must have struck most naturalists as a strange 

 anomaly that, Jboth with animals and plantSj some species 

 of the same family and even of the same genus, though 

 agreeing closely with each other in their whole organiza- 

 tion, are hermaphrodites, and some unisexual. But if, 

 in fact, all hermaphrodites do occasionally intercross, the 

 difference between them and unisexual species is, as fa»' 

 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 occasional intercross between distinct individuals 

 is a very general, if not universal, law of nature. 



Circumstances Favorable 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 favor- 

 able. A large number of individuals, by giving a better 

 chance within any given period for the appearance of 

 profitable variations, will compensate for a lesser amount 

 of variability in each individual, and is, 1 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 

 I all organic beings are striving to seize on each place in 

 the economy of nature, if any one species does not be- 

 come modified and improved in a corresponding degree 

 ^>with its competitors, it will be exterminated. Unless 



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