

200GENESIS 



The development of animal forms, as we learn from 

 a study of the fossils, is not a reversible phenomenon. 

 Once specialization has begun it always continues — 

 by a process of progressive modification and subtrac- 

 tion — in the same general direction, becoming more 

 and more extreme. There is never any retrogres- 

 sion. 



Heretofore this principle has been applied only to 

 more or less restricted groups of animals. But there 

 is no reason to suppose that it is not applicable to 

 animals as a whole. 



If we attempt to apply it to animals as a whole — 

 that is, to the various major groups or phyla — we at 

 once discover some most interesting facts. Most 

 important of these facts is that we find it wholly 

 impossible to arrange the phyla in any sort of a line 

 showing progressive subtraction and no addition — 

 such a line as is so evident, for instance, in the horses. 



Each phylum includes animals of a definite struc- 

 tural type of complex which is widely different from 

 the structural type or complex characteristic of the 

 animals in every other phylum. These structural 

 complexes characteristic of the several phyla differ 

 from each other by modification of fundamental 

 structures, including the addition of some features and 

 the subtraction of others. 



Thus the echinoderms have acquired a radial sym- 

 metry apparently through the subtraction or loss of 

 half of each segment of the body, and in addition have 

 lost the nephridial system — at least in its usual form 

 — and all trace of chitinous structures; the annelids 



[105] 



