THE NEW EVOLUTION 



sible to interpret the vertebrates as combining the features found 

 in all of them and therefore as occupying, as the most highly 

 perfected of all animal types, the center of the figure. 



In the vertebrates we are able to recognize the segmentation of 

 the cestodes, annelids and cephalochordates, combined with the 

 coclomic structure first indicated in the flukes, both enclosed in 

 the undivided body of the turbellarians and nematodes. If the 

 vertebrate limbs may be compared to budded units recalling cer- 

 tain highly reduced and specialized units in tunicates or polyzoans, 

 the comparison is complete. 



In these various recombinations of structural features leading 

 to greater and greater structural complexity, numerous secondary 

 features, such as visual and other sense organs, appendages of 

 different kinds, diverticula and other outgrowths from the enteric 

 canal, chitinous and calcareous skeletons, and others, became 

 enormously developed and specialized in correlation with the 

 increasing bodily efficiency resulting from the progressive im- 

 provement in the structural balance. 



In this exposition of the development of the fundamental ani- 

 mal forms^ — of the original type representing each of the major 

 groups or phyla — no claim is made that the last word has been 

 spoken in regard to the placing of each phylum in relation to 

 the others. But it is believed that the diagrammatic representa- 

 tion here given is in all essential features correct. 



There can be no talk of any direct relationship between the 

 adult forms in one phylum and the adult forms in any other. 

 There is not now, and we have no evidence that there ever was, 

 any intergradation between the phyla as represented by post- 

 embryonic forms. 



All of the evidence points to the origin of the phyla through 

 modifications of the regular geometrical developmental course 

 which took place in the gastrula, or approximately at the gas- 

 trula, stage, each departure from the regular course resulting in a 

 different bilateral animal form. 



This means that all animal types arose through various recom- 

 binations of features which are already present in the gastrula, 

 and further that the animal types differ from each other not by 

 the addition of certain features but, on the contrary, by the sub- 

 traction or repression of a varying number of the features which 

 are inherent in, or an integral part of, the perfect gastrula, this 



