CHAPTER V 



THE ANCESTRY OF THE VERTEBRATES 



BEFORE it was clearly recognized that the In- 

 vertebrate phyla were separated from one another 

 by fundamental differences, it was customary to 

 regard the distinction between Invertebrate and 

 Vertebrate animals as constituting the great cleav- 

 age or dichotomy of the animal kingdom. At the 

 present time, although we may fully admit that 

 among Invertebrate animals cleavages of equal 

 magnitude may exist, yet the fact that among the 

 Vertebrata are included all the highest developments 

 of animal life culminating in man himself, has invested 

 and still invests the question of the origin of the 

 Vertebrate stock with peculiar interest. From what 

 existing phylum of Invertebrata, if from any, did the 

 Vertebrate type take origin ? That is the question 

 which in the past and present has been debated with 

 vigorous and sometimes almost ecstatic fervour. The 

 true Vertebrates include all those animals which 

 possess a dorsal tubular central nervous system, a 

 segmented backbone or vertebral column enclosing 

 the nerve-cord, a closed vascular system containing 



