NATURAL SELECTION SINCE DARWIN 71 



stage of the life struggle; advantages are not 

 grouped together and they may be compensated by 

 certain drawbacks. This argument carries weight 

 and shows that in the discussion of this question the 

 issue must not be too simplified and that we must be- 

 ware of reasoning mathematically, assuming all other 

 things to be equal. This method of reasoning can be 

 applied in experiments well prepared and conducted 

 with the utmost care, but it cannot be applied to phe- 

 nomena such as we observe in living nature. 



