EVOLUTION 



343 



evolution. When some leading biologist attacks the theory of natural 

 selection, as has been done time and again, some one may proclaim that 

 the doctrine of evolution has been relegated to history. Nothing is 

 farther from the truth. It is only the theories that would account for 

 evolution, or theories that concern the course which evolution has taken, 

 that are still the subject of controversy. 



The history of the evolution idea in the last forty or fifty years has 

 been the accumulation of new facts in support of it, the development of 

 theories to account for it, the grouping of animals on the basis of the 

 relationship implied in evolution, and the application of corollaries of 

 evolution to other branches of biology. Since a discussion of these later 



Fig. 235.— Thomas Henry Huxley, 1825-1895. 



developments often involves the subject matter of the other branches, any 

 further references to the history of the doctrine of evolution that seems 

 desirable will be made in connection with the evidence of evolution and 

 the theories of its causes in the following pages. 



Evidence of Evolution. — Before proceeding to the implication? of evo- 

 lution, it will be profitable to review the evidence on which belief in the 

 changeability of species is based. The best evidence of any process is to 

 witness it going on. If species can be shown to changs while under ob- 

 servation, the best possible evidence of evolution has been obtained. 

 Whenever one or more members of a species possess a characteristic not 

 found in other members of the species in the same generation, or in pre- 

 ceding generations, evolution has occurred, provided the new feature is of 

 such a nature as to be inherited. The heritability of the new feature is 

 insisted upon, as otherwise the change would be temporary. There are 



