PREFACE. 3 



that heredity is an acquired force, and that, normally 

 or originally, unlike produces unlike. 



It may be well to state what are the chief lines 

 of proof of evolution as they appeal to the author. 

 A priori reasons for belief in the hypothesis are the 

 two facts, that there must be struggle for existence 

 from the mere mathematics of propagation, and that 

 there have been mighty changes in the physical char- 

 acter of the earth. These facts argue that organisms 

 must either have changed or perished. To me, the 

 chief demonstrative reason for belief in evolution is 

 the fact that plants and animals can be and are modi- 

 fied profoundly by the care of man. In fact, I should 

 be convinced that the organic creation is an evolution 

 if I had no other proof than this. But the proofs 

 are abundant: 



1. Those afforded by paleontology. 



2. Those of embryology. 



3. Those of comparative anatomy and structure. 



4. Resemblances of types, which allow the objects 

 to be classified. If species were specially created, 

 there would have been no relationships. 



5. The successive increase in complexity and dif- 

 ferentiation, or divergence, in this classification, or the 

 growth of the "tree of life." 



6. The fact of adaptation to environment. 



7. The vagaries of distribution, (See Essay XV.) 



