178 DARWINISM chap. 



l^hysiological peculiarity. Thus, incipient species -which were 

 infertile together would have an advantage over other 

 incipient si)ecies which were fertile, and, whenever the 

 struggle for existence became severe, would prevail over them 

 and take their place. Such infertility, being correlated with 

 constitutional or structural differences, would j^robably, as 

 already suggested, go on increasing as these differences 

 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 well 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 probable, the fact of variations in fertility 

 occurring in correlation with variations in habits, colour, or 

 structure, we 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 would tend to die out almost as fast as 

 it was 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 individuals of the two forms in definite areas, it would 

 be preserved by natural selection, and the portion of the 

 varying species thus affected Avould increase at the expense of 

 those portions which were more fertile when crossed. Each 

 further variation towards infertility between the two forms 

 would 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, Ave have 

 seen that if several competing species in the same area were 

 being simultaneously modified, those between whose varieties 

 infertility arose would have an advantage over those whose 

 varieties remained fertile inter se, and would ultimately sup- 

 plant them. 



The preceding argument, it will be seen, depends entirely 

 upon the assumption that some amount of infertility char- 

 acterises the distinct varieties which are in process of 

 differentiation into species ; and it may be objected that of 



