THE ORIGIN OF ANIMAL SPECIES 457 



in Part I, animals are divided according to their struc- 

 tural resemblances into groups of varying degrees of 

 affinity, as branch, class, order, genus, and species. A 

 pictorial representation of the scheme of classification 

 would have the form of a genealogical tree (Fig. 197), 

 the relative positions of whose branches would indicate 

 the degree of relationship among the different groups. 

 This scheme implies that there is actual genetic relation- 

 ship and community of descent of all animal forms, the 

 Metazoa from the Protozoa, the air-breathing vertebrates 

 from fishlike ancestors, the birds and mammals from 

 reptilian prototypes. There is thus evolution of the 

 more complex from forms of simpler structure. The 

 underlying principle of classification is heredity, or 

 community of descent, as indicated by family likeness. 

 After trying many other structural features as means of 

 classification systematic zoologists found that the surest 

 guides in determining relationship are frequently organs 

 of little or no assignable physiological importance. 

 There was no explanation of this seeming paradox 

 until it was seen that, according to Darwin's theory, 

 such organs are not likely to undergo change, since they 

 are apparently not of vital importance to the possessor 

 and are handed down through successive generations 

 little, if at all, modified, however much the rest of the 

 body may have changed in becoming adapted to its 

 environment. 



(2) Evidence from Morphology. The comparative 

 anatomy of animals furnishes some of the strongest 

 evidence in favor of the theory of evolution. Note, for 

 instance, the increasing complexity of form and function 

 as the various branches are passed in review the single- 

 celled Protozoa, showing colony formation in the higher 

 orders with differentiation in form and function among the 

 members, as Zoothamnium ; the loosely cellular sponges, 



