518 



GENERAL CONSIDERATIONS 



586. Evidences of Evolution. — Among the evidences which support 

 the evolutionary conception is the fact that variation is seen everywhere 

 among organisms and that many of these variations are clearly trans- 

 missible to succeeding generations. Other evidences are that it is pos- 

 sible under cultivation to modify animals in certain definite directions 

 which are advantageous, and that under experimental conditions in the 

 laboratory animals have been caused to undergo changes which have been 

 inherited. 



The facts that all protoplasm is practically the same in character, that 

 metabolism is carried on by all living organisms, that such phenomena as 



Fig. 31G. — Charles Darwdn, 1809-1882. {From Shrill, "Prmciples of Animal Biology." 

 Photo by Leonard Darwin in University Magazine. By the courtesy of McGraw-Hill Book 

 Com,pany, Inc.) 



mitosis, gametogenesis, and embryogeny are phenomena which, in 

 general , always occur in a similar fashion are all further evidences of 

 relationship due to common origin. 



Comparative anatomy furnishes numerous evidences of evolution, 

 many of which can be summed up under the general head of homology. 

 The existence of homologous structures, which is very prevalent among 

 animals, indicates relationship and a common ancestry. Some particular 

 examples are the homologies which exist in the series of vertebrate skulls 

 from fish to man, the uniformity of plan in the vertebrate limb, and the 

 common plan of structure which shows itself in the brains of vertebrates. 

 Another evidence derived from comparative anatomy is the existence in 

 certain animals of vestigial parts and organs (Fig. 317), together with the 

 fact that in other animals which from their general structure are appar- 



