CHAPTER IV. 

 Evidences of Physiological Selection. 



I WILL now give an outline sketch of the evidences 

 in favour of the theory which has been set forth in 

 the preceding chapter, stating first what is the nature 

 of the verification which it requires. 



The theory is deduced from a highly general 

 association between distinctive specific characters 

 of any kind and a relatively constant specific 

 character of a particular kind — namely, sexual 

 exclusiveness. For it is from this highly general 

 association that the theory infers that this relatively 

 constant specific character has been at least one of 

 the needful conditions to the development of the 

 other specific characters with which it is found 

 associated. Hence the necessary verification must 

 begin by showing the strength of the theory on these 

 merely deductive, or antecedent, grounds. It may 

 then proceed to show how far the facts of organic 

 nature corroborate the theory in other and inde- 

 pendent ways. 



First, let it be carefully observed that here we have 

 to do only with the fact of selective fertility, and with 

 its consequences as supposed by the theory : we have 



