OBVERSE A PRIORI PRINCIPLE. 411 



the requirements considered under each, aspect. The two 

 necessities correspond. 



We might rest on these deductions and their several corol- 

 laries. Without going further we might with safety assert 

 the general truths that, other things equal, advancing evolu- 

 tion must be accompanied by declining fertility ; and that, in 

 the highest types, fertility must still further decrease if 

 evolution still further increases. We might be sure that if, 

 other things equal, the relations between an organism and its 

 environment become so changed as permanently to diminish 

 the difficulties of self-preservation, there will be a permanent 

 increase in the rate of multiplication ; and, conversely, that a 

 decrease of fertility will result where altered circumstances 

 make ;self- preservation more laborious. 



But we need not content ourselves w T ith these a priori 

 inferences. If they are true, there must be an agreement 

 between them and the observed facts. Let us see how far 

 eiich an agreement is traceable. 



