A PRIORI PRINCIPLE. 403 



in this, that what is a temporary relation in the ono is a per 

 manent relation in the other. 



323. For the moment it does not concern us to inquire 

 what is the origin of this permanent relation. That which 

 we have now to note, is simply that in some way or other 

 ihere must be established an inverse proportion between the 

 power to sustain individual life and the power to produce 

 new individuals. Here it is enough for us to recognize this 

 as a necessary truth. Whether or not the permanent rela 

 tion is self-adjusting in long periods of time, as the tempo 

 rary relation is self-adjusting in short periods of time, is a 

 separate question. The purpose of this chapter is fulfilled by 

 showing that such a permanent relation must exist. 



But having recognized the a priori principle that in races 

 which continuously survive, the forces destructive of race 

 must be equilibrated by the forces preservative of race ; and 

 that supposing these are constant, there must be an inverse 

 proportion between self-preservation and race- preservation ; 

 we may go on to inquire how this relation, necessary in 

 theory, arises in fact. Leaving out the untenable hypothesis 

 of a supernatural pre-adjustment, we have to ask in what 

 way an adjustment comes about as a result of Evolution. 

 Is it due to the survival of varieties in which the proportion 

 of fertility to mortality happens to be the best ? Or is the 

 fertility adapted to the mortality in a more direct way P To 

 these questions let us now address oursel ves. 



