Evolution. 241 



organic form, in exactly the same way as a rhombohedron 

 of calcite or a rhombic dodecahedron of garnet is a definite 

 chemical compound, with its proper crystalline form. All 

 that the analogy is intended to convey is that variations 

 seem, under certain circumstances, to be definite and stable, 

 and may possibly combine rather than commingle. 



Summary and Conclusion. 



It only remains to bring this chapter to a close with a 

 few words of summary and conclusion. 



The diversity of animal life must first be grasped. We 

 believe that this diversity is the result of a process or 

 processes of evolution. Evolution is the term applied to 

 continuity of development. It involves adaptation ; and 

 adaptation to an unchanging environment may become 

 more and more perfect. But the environment to which 

 organisms are adapted also changes. Where the change is 

 in the direction of complexity, we have elaboration ; where 

 it is in the direction of simplicity, we have degeneration. 

 Of these elaboration is the more important. It involves 

 both a tendency to differentiation giving rise to indi- 

 viduality, and a tendency to integration giving rise to 

 association. Continued elaboration is progress ; and this 

 is opposed to degeneration. 



The factors of evolution fall under two heads origin 

 and guidance. The origin of variations lies in mechanical 

 stresses, and chemical or physical influences. Whether 

 these act on the body (and are transmitted by inheritance) 

 or only on the germ, is. a question which divides biologists 

 into two schools. In the latter case all variations are fortui- 

 tous; in the former the development of tissue-variations 

 has been in direct response to the physical or chemical 

 influences. There are, however, in any case fortuitous 

 combinations of variations. 



Whether use and disuse are factors of origin is also a 

 debatable point. Those who believe that physical influences 



