422 Causes and Course of Organic Evolution 



of the nemerteans is comparable in situation (and develop- 

 ment?) with the chorda dorsalis of vertebrates." 



The arguments he adduced (754:349; 135:470; 136: Ul) 

 were alike weighty and suggestive, but they have hitherto 

 stimulated to no marked advance. Somewhat strangely also 

 Hubrecht, though greatly improving his original position by 

 additional embrj^ological and histological evidence, does not 

 seem to have followed out his fruitful suggestions by study 

 either of the lower or of appropriate higher animals. This 

 we believe was probably due largely to the predominating 

 attention given by other zoologists at that time to the Enter- 

 opneusta, the Urochordata or ascidians, and the Cephalo- 

 chordata or Amphioxvs group, all of which seem to be decidedly 

 lateral or even degraded offshoots from the main ascending 

 line. 



To the TVTiter, Hubrecht's position forms one of the most 

 far-reaching and brilliant steps ever taken in zoological sci- 

 ence. F. M. Balfour had advocated a nemertean ancestry 

 for vertebrates, while Lankester decidedly favored Hubrecht's 

 view. But the past quarter century has witnessed no further 

 advance. The writer will therefore try to advance the position 

 further, in hope that zoologists may take it up in its many 

 fruitful ramifications. 



From the standpoint of advantageous organization in the 

 struggle for existence, the formation of the proboscis and pro- 

 boscis sheath has two high recommendations in the abstract. 

 From early dorsal invagination of the whole structure, in some 

 now lost type of the rhabdocoel turbellarians, it came to lie 

 directly between and against the dorsal nerve-mass and to be 

 increasingly stimulated by the latter; it thus placed the organ- 

 ism in intimate and delicate relation to its environment. In 

 other woids, between the brain as the evolving cognito-cogitic 

 center and the en\dronment of the organism, the most direct 

 and sensitive pathway possible was established. This, we 

 consider, explains why the nemerteans gave off phyla that in 

 time progressed by selective survival to higher and successful 

 stages of organization, alongside armored animals of the ar- 

 thropod and molluscan types that have largely succumbed. 



Second, the increasingly perfect protrusion and retraction 

 of the proboscis into a muscular wall or sheath, such as we 

 see in its primitive form amongst turbellarians, started a nu- 

 cleus or center of resistance for the whole body, such as no 

 other parallel group possesses. Thus the sheath, originally 

 a dorsal mesodermal or more likely endodermally derived 

 tubular investment to the proboscis, became a cylindrical 



