284 MORPHOLOGY 



thing, and to explain it is a very different thing. The well-known 

 names associated with the doctrine of evolution often are thought of as 

 the names of men who may be called authors of the theory of evolution; 

 but they are really men who have proposed explanations of evolution. 

 For example, Darwin, perhaps more than any other evolutionist, is 

 spoken of as the author of the doctrine of organic evolution; but if his 

 explanation and every other explanation should be disproved, the fact 

 of evolution would still remain to be explained. No proposed explana- 

 tion of evolution is entirely satisfactory, but biologists are daily becom- 

 ing more convinced of the truth of evolution. 



With this distinction between the fact of evolution and its explanation 

 made clear, it will be possible to outline briefly the conspicuous expla- 

 nations that have been offered. The problem to be solved is how new 

 forms may arise from old ones, which is the problem of the origin of 

 species. 



Environment. — Perhaps the oldest explanation of organic evolution, 

 based upon observation, is that plants and animals may be changed 

 by their environment. Such facts as the seasonal changes in the plum- 

 age of birds and in the covering of mammals, and also the changes in 

 plants in relation to their environment, suggested that plants and ani- 

 mals are plastic and can be molded by a changing environment. This 

 explanation was offered, during the last decade of the eighteenth cen- 

 tury, by Erasmus Darwin of England, St. Hilaire of France, and 

 Goethe of Germany. It was assumed that any change induced by en- 

 vironment would be transmitted to the offspring, to be retained so long 

 as the environment remained constant. 



It is evident that organisms respond more or less in certain ways to 

 decided changes in the environment, but such direct responses are re- 

 garded generally as too superficial and fluctuating to account for the 

 production of new forms. The influence of environment, however, 

 while insufficient to explain organic evolution, is still recognized as an 

 important factor of the problem, whose value may vary widely. If 

 there are such things as "ecological species," their origin is due by 

 definition to environment. 



Use and disuse. — In the early part of the nineteenth century, Lamarck 

 offered an explanation of evolution, which he called " appetency," 

 meaning the effect of desire. The theory is better known as Lamarck- 

 ism, and it "has strong defenders, in modified form, to the present day. 

 It is really the effect of use and disuse. It is well known that persistent 



