OBVERSE A PRIORI PRINCIPLE. 431 



use to counterbalance these destroying forces ; and in propor- 

 tion as much of it is expended in preserving the individual, 

 little of it can be reserved for producing new individuals, 

 and vice versa. There is thus complete accordance between 

 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 evolu- 

 tion still further increases. We might be sure that if, other 

 things equal, the relations between an organism and its en- 

 vironment 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 with these a priori 

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

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

 such an agreement is traceable. 



