LIFE. ITS l^HYSICAL BASIS AXD SIMPLEST EXPniOSSIu.N oO 



Fig. 23. — Pnndin-init np., a rolnnial 

 pri>t(iZ(><ji». UliKlily iniiimifi(xj.) 



of fishes, amphibians, reptiles, 

 birds, and mammals. 



In all the subdivisions of tiic 

 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 finuli\ ui 

 sub-species, varieties, and individuals. I*^ach one of tlu^sc 



names refers p;:- 

 marily to a spocial 

 line or mode of 

 differentiation and 

 at the same time 

 refers to tlie fact 

 that the members 

 of each of tlie.><e 

 (1 i f f (^ r ( ' n t i a t e cl 

 groups are genet- 

 ically related to 

 each other. t!i:it 

 is, related l>y 

 blood, by actu;'.l 

 ancestral descent. 

 All these diffcr- 

 enliated groups 

 indicate diverging 

 lines of cvolutinn. 

 some of flieni 

 short, ami but 



Fig. 24.— a fresh-water polyp, Hiidm vulgaris: A. in ex- sliglltly diverg*'nt 



tended condition and in contraofod condition; H, cross from tllC MlJlih 



section of body, showing the two layers of cells which .. from Nvlucll 

 make up the body wall. 



