116 EVOLUTION AND ANIMAL LIFE 



to be exactly this: we cannot explain to any general satisfac- 

 tion species-forming and evolution without the help of some 

 Lamarckian or Eimerian factor; and on the other hand, we 

 cannot assume the actuality of any such factor in the light of 

 our present knowledge of heredity. The discovery of the 

 "unknown factors of evolution" should be the chief goal of all 

 present-day biologic investigation. 



