ORGANIC EVOLUTION 39 



species, branching and rebranching from the main 

 trunk. The existence of structures, so graduated 

 as to render such an arrangement possible, is in 

 itself suggestive of a common relationship and 

 origin. 



2. Evolution is suggested by the similarities 

 and homologies of structure found throughout the 

 animal kingdom. Some of these similarities and 

 homologies have already been mentioned. They 

 are everywhere — remoter and more fundamental, 

 some of them, others closer and more detailed. 

 To the untrained mind, which sees surfaces only, 

 and not even surfaces well, the animal world is an 

 interminable miscellany of forms. But to the 

 biologist, who looks deeper and w^ith immense 

 acumen over the whole field of animal life, there 

 are only seven or eight different types of structure 

 in the entire animal world. These seven or eight 

 types correspond with the primary classes, or 

 phyla, into which animals are divided, viz., pro- 

 tozoa, sponges, celenterates, echinoderms, worms, 

 mollusks, arthropods, and vertebrates. However 

 widely the members of each of these great groups 

 may differ among themselves in colour, size, 

 habits of life, and the like, the members of each 

 group all resemble each other fundamentally. 

 Moles differ from monkeys, bats from men, and 

 birds from crocodiles and toads. They differ 

 enormously. But they are all vertebrates with 

 red blood, double body cavities, backbones, two 

 pairs of limbs, and five fingers on each limb. 

 When they are looked at superficially, there is 



