88 



ORGANIC EVOLUTION 



The phenomena of comparative anatomy in their bearing 

 upon the theory of evolution. 



Classification. 



All are familiar with the fact that animals and plants are 

 of very many different sorts, and that the different kinds show 



very different degrees of 

 complexity in their or- 

 ganization. We give ex- 

 pression to these facts in 

 our classification of ani- 

 mals and plants. Forms 

 which are closely similar 

 almost to the point of 

 identity we call members 

 of the same species. For 

 example, while . hardly 

 any, if any, two robins 

 are so similar that we 

 cannot detect some dif- 

 ferences between them, 

 still all robins quite 

 closely conform to the 

 same type, and their mutual differences are so slight that 

 without hesitation we group them together in one species. 

 We see the same thing among plants. Such of our 

 common blue violets as have rounded, heart-shaped, slightly 

 pointed leaves, and scentless blue flowers of large size, 

 having also very much shortened stems, we class under 

 the one species Viola cucullata (Fig. n). (There are other 

 characters of the species besides those mentioned by which 



FIG. ii. Viola cucullata. From Britton and 

 Brown's Illustrated Flora of the Northern States and 

 Canada, by the courtesy of the authors and of Charles 

 Scribner's Sons. 



