178 DARWINISM cuav. 



physiological peculiarity. Thus, incipient species which were 

 infertile together Avould have an advantage over other 

 incipient species which were fertile, and, whenever the 

 struggle for existence became severe, would prevail over them 

 and take their place. Such infertility, being correlated Anth 

 constitutional or structural differences, would probably, as 

 already suggested, go on increasing as these difierences 

 increased ; and thus, by the time the new species became 

 fully differentiated from its parent form (or brother variety) 

 the infertility might have become as Avell marked as we 

 usually find it to be between distinct species. 



This discussion has led us to some conclusions of the greatest 

 importance as bearing on the difficult problem of the cause of 

 the sterility of the hybrids between distinct species. Accept- 

 ing, as highly probaljle, the fact of variations in fertility 

 occiu-ring in correlation Avith variations in habits, colour, or 

 structure, Ave see, that so long as such variations occurred only 

 sporadically, and affected but a small proportion of the in- 

 dividuals in any area, the infertility could not be increased by 

 natural selection, but Avould tend to die out almost as fast as 

 it Avas produced. If, however, it was so closely correlated 

 with physical variations or diverse modes of life as to 

 affect, even in a small degree, a considerable proportion of 

 the indiAdduals of the tAvo forms in definite areas, it Avould 

 be preserved by natural selection, and the portion of the 

 varying species thus affected Avould increase at the expense of 

 those portions Avhich Avere more fertile Avhen crossed. Each 

 further A^ariation toAA^ards infertility between the two forms 

 Avould be again preserved, and thus the incipient infertility 

 of the hybrid offspring might be increased till it became so 

 great as almost to amount to sterility. Yet further, we have 

 seen that if several competing species in the same area were 

 being simultaneously modified, those betAveen AA'hose varieties 

 infertility arose would have an advantage over those Avhose 

 varieties remained fertile inte?' se, and Avould ultimately sup- 

 plant them. 



The preceding argument, it Avill be seen, depends entirely 

 upon the assumption that some amount of infertility char- 

 acterises the distinct A'arieties AA'hich are in process of 

 differentiation into sj^ecies ; and it may be objected that of 



