176 DARWINISM chap. 



greatly assisted by two other important factors in the 

 problem. It has already been shown that, with each 

 modification of form and habits, and especially with modifica- 

 tions of colour, there arises a disinclination of the two forms 

 to pair together; and this would produce an amount of 

 isolation which would greatly assist the specialisation of the 

 forms in adaptation to their different conditions of life. 

 Again, evidence has been adduced that change of conditions 

 or of mode of life is a potent cause of disturbance of the 

 reproductive system, and, consequently, of infertility. We 

 may therefore assume that, as the two forms adopted more 

 and more different modes of life, and perhaps acquired also 

 decided peculiarities of form and coloration, the infertility 

 between them would increase or become more general ; and as 

 we have seen that every such increase of infertility would 

 give that portion of the species in which it arose an advantage 

 over the remaining portions in which the two varieties were 

 more fertile together, all this induced infertility would main- 

 tain itself, and still further increase the general infertility be- 

 tween the two forms of the species. 



It follows, then, that specialisation to separate conditions 

 of life, differentiation of external characters, disinclination to 

 cross-unions, and the infertility of the hybrid produce of these 

 unions, would all proceed pari passu, and would ultimately 

 lead to the production of two distinct forms having all the 

 characteristics, physiological as well as structural, of true 

 species. 



In the case now discussed it has been supposed, that some 

 amount of general infertility might arise in correlation with 

 the different modes of life of two varieties or incipient 

 species. A considerable body of facts already adduced 

 renders it probable that this is the mode in which any 

 widespread infertility would arise ; and, if so, it has been 

 shown that, by the influence of natural selection and the 

 known laws which affect varieties, the infertility would be 

 gradually increased. But, if we suppose the infertility to 

 arise sporadically within the two forms, and to affect only a 

 small proportion of the individuals in any area, it will be 

 difficult, if not impossible, to show that such infertility would 

 have any tendency to increase, or would produce any but a 



