LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION 35 



FIG. 23. Pandorina sp., a colonial 

 protozoon. (.Highly magnified.) 



of fishes, amphibians, reptiles, 

 birds, and mammals. 



In all the subdivisions of the 

 main groups there are also to be 

 recognized differentiated and di- 

 vergent lesser lines of descent, and 

 within these still lesser ones. 

 While, as already noted, the main 

 divisions of the animal kingdom 

 are called phyla and the divisions 

 of the phyla, classes, the subdi- 

 visions of the classes are usually 

 called orders. The next subdi- 

 vision is that into families, each 

 in. turn being a cluster of genera. 



The genera are composed of species and the species finally of 

 sub-species, varieties, and individuals. Each one of these 



names refers pri- 

 marily to a special 

 line or mode of 

 differentiation and 

 at the same time 

 refers to the fact 

 that the members 

 of each of these 

 differe n t i a t e d 

 groups are genet- 

 ically related to 

 each other, that 

 is, related by 

 blood, by actual 

 ancestral descent. 

 All these differ- 

 entiated groups 

 indicate diverging 

 lines of evolution, 

 some of them 

 short, and but 



FIG. 24. A fresh-water polyp, Hydra vulgaris: A, in ex- slightly divergent 



tended condition and in contracted condition; B, cross from the main 

 section of body, showing the two layers of cells which , . 1 



make up the body wall. lme fr m which 



