CLASSIFICATION AND PHYLOGENY 231 



ancestral Protista are still represented by descendants which 

 probably differ but little from their remote progenitors and which 

 have not yet reached the level of either plants or animals, and 

 there are still living unicellular forms in abundance which, though 

 they may perhaps be distinguished as plants or animals, have 

 never been able to learn the secret of forming multicellular 

 bodies. So it is with most of the great ancestral groups ; they 

 are represented still by descendants which have undergone com- 

 paratively little change in structure since remote geological 

 periods. The algae are still algae, the ccelenterates are still 

 coelenterates and the fishes are still fishes, though each of these 

 great groups has in past time given rise to descendants which 

 have gradually become modified into higher types. 



Many subordinate groups, such as the trilobites and ammonites 

 and the winged reptiles of the secondary period, have of course 

 become extinct during past geological ages, but the fact remains 

 that even at the present day there still exist on the earth organic 

 types which represent, in a more or less unmodified form, all the 

 principal stages of organic evolution, and thus it is that, even if 

 we had only living plants and animals to consider, our classifica- 

 tion of the organic world would still assume a tree-like form, 

 with the simplest organisms at the bottom and the most complex 

 at the top of the tree. It would be difficult indeed to explain 

 this fact in accordance with the theory of special creation and 

 immutability of species. 



The monopodial branching of the organic tree and the relation 

 which the natural classification of existing animals bears to the 

 phylogenetic or ancestral series is diagrammatically represented 

 in Fig. 87. On the right hand side are shown some of the 

 principal stages in the evolution of vertebrates from ancestral 

 Protista, and at the top of the tree are shown the existing groups 

 by which these stages are actually represented at the present 

 day. Only a few stages are represented and the great majority 

 of the lateral branches of the tree are supposed to have been 

 lopped off. 



