^^^^ THE NEW EVOLUTION ®l 



the animal phyla or major groups may be arranged in 

 five successive series of four each, the outermost four 

 being the four partially radial types mentioned. The 

 exact center of the figure is occupied by the verte- 

 brates, which combine the characters of the four 

 groups immediately surrounding them (cephalochor- 

 dates, balanoglossids, cephalodiscids and tunicates) 

 but are not more closely related to any one of these 

 than they are to the other three (fig. E, p. 2.54). 



Such a figure shows each phylum as related more or 

 less equally to four others, and more distantly to all 

 the rest. As we pass from the outer to the inner 

 series we find that the phyla become more and more 

 complex and, because of their increasing complexity, 

 seemingly less and less widely differentiated from 

 each other. 



But how could such a curiously complicated inter- 

 relationship come into being? How could any single 

 type of animal be related more or less equally to four 

 others, and why should the more complexly organized 

 types — the cephalodiscids, balanoglossids, cephalo- 

 chordates, tunicates and vertebrates — be less widely 

 different from each other than the more simply or- 

 ganized types? 



The answer is a simple one. Since we have not the 

 slightest evidence, either among the living or the 

 fossil animals, of any intergrading types falling be- 

 tween the major groups it is a fair supposition that 

 there never have been any such intergrading types. 



We find twenty definite structural complexes which 

 are apparently without any direct relation to each 



[196] 



