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. 



