CHAPTER IV. 

 DIFFICULTIES OF INDUCTIVE VERIFICATION. 



329. Were all species subject to the same kinds and 

 n mounts of destructive forces, it would be easy, by comparing 

 different species, to test the inverse variation of Individuatioa 

 and Genesis. Or if either the power of self-preservation or 

 the power of multiplication were constant, there would be 

 little difficulty in seeing how the other changed as the 

 destroying foi-ces changed. But comparisons are nearly 

 always partially vitiated by some want of parity. Each 

 factor, besides being variable as a whole, is compounded 

 of factors that are severally variable. Not simply is the sum 

 of the force? destructive of race different in every case ; and 

 not simply are both sets of forces preservative of race unlike in 

 their totalities in every case ; but each is made up of actions 

 that bear such changing proportions to one another as to 

 prevent any positive estimation of its amount. 



Before dealing with the facts as well as we can, it will be 

 best to glance at the chief difficulties ; so that we may see the 

 kind of verification which is alone possible. 



330 Either absolutely, or relatively to any species, 

 every environment differs more or less from every other. 



There are the unlikenesses of media air, water, earth, 

 organic matter ; severally involving special resistances to 

 movement, and special losses of heat. There are the con- 



