466 COMPARATIVE ZOOLOGY 



scendants. On this theory Darwin accounted for the 

 development of antlers, the beautiful colors of birds, 

 fishes, and insects, and the calls of various animals. 



While evolution has come to be regarded as a fact of 

 as much certainty as gravitation, and natural selection, 

 with variation and heredity, to be accepted by many 

 naturalists as the process by which evolution is brought 

 about, not all are agreed as fo the importance to be 

 attributed to the Darwinian factor, i.e. natural selection. 

 Darwin himself regarded it as "the main but not the 

 exclusive means of modification." Search for additional 

 means has been made and is still being prosecuted. Thus 

 consciousness is claimed to be a controlling agent in the 

 use and disuse of organs, in the adoption of new habits, 

 in the selection of environment, in the choice of mates, 

 and so on. Isolation due to the geographical separation 

 of individuals or of species, or to the inability of forms 

 to interbreed {physiological isolation) has been suggested 

 as another cause of modification. Still another factor, 

 organic selection, has been proposed. It is claimed for 

 this that the adoption of a new habit by an animal will 

 lead to the development of structures adapted to the 

 habit, and thus produce changes in specific differences. 



The most important addition to the philosophy of 

 organic evolution made since Darwin is Weismann's 

 theory of the continuity of the germ plasm, which 

 maintains, supported by facts of observation, that the 

 essential germinal substance is transmitted from gen- 

 eration to generation through the reproductive cells. 

 Whether or not this material the bearer of heredity 

 may be influenced by structural and physiological 

 changes occurring in the species, and thus be trans- 

 mitted to descendants, is a question which has not yet 

 been definitely answered. 



