PRELIMINARY. 



IN Part II., which treats of the causes of variations, 

 I propose to cite examples of the direct modifying 

 effect of external influences on the characters of indi- 

 vidual animals and plants. These influences fall nat- 

 urally into two classes, viz., the physico-chemical 

 (molecular), and the mechanical (molar). The modi- 

 fications so presented are supposed to be the result of 

 the action of the causes in question, continued through- 

 out geologic time. To the two types of influence 

 which thus express themselves in evolution, I have 

 given the names Physiogenesis^ and Kinetogenesis. 

 The inheritance of character is assumed in this sec- 

 tion, and the reason for so doing will be considered 

 later, in the third section of this book. 



In the animal kingdom we may reasonably suppose 

 that kinetogenesis is more potent as an efficient cause 

 of evolution than physiogenesis. In the vegetable 

 kingdom it is quite evident that evolution is more 

 usually physiogenetic than kinetogenetic. Atmospheric 

 and terrestrial conditions play a major role in the 



1 " The Energy of Evolution," American Naturalist, March, 1894. "The 

 Origin of Structural Variations " in New Occasions, Chicago, May, 1894. C. H. 

 Kerr & Co. 



