86 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



a process of degeneration, and therefore of compara- 

 tively modern origin. The question, in brief, is, 

 whether these creatures presenting these features be 

 primitive ancestors or degenerate descendants. 



A great deal of light has been happily thrown on 

 this question, as regards the Vertebrata, by the recent 

 work done in North American paleontology. The lines 

 of descent of many of the minor groups have been 

 positively determined, and the phylogenetic connec- 

 tions of most of the primary divisions or classes have 

 been made out. The result of these investigations has 

 been to prove that the evolution of the Vertebrata has 

 proceeded not only on lines of acceleration, but also 

 on lines of retardation. That is, that evolution has 

 been not only progressive, but at times retrogressive. 



The Amphioxus (genus Branchiostoma) is generally 

 regarded as the ancestral vertebrate. There are many 

 reasons why this position must be accepted, although 

 it possesses a few secondary modifications. Whether 

 Branchiostoma derived its descent from an annelid 

 worm, or from a tunicate, is a vexed question. Brooks 1 

 remarks as to this, "Up to this point I believe that 

 the ancestral history of the tunicates was identical with 

 that of the vertebrates ; for the hepatic ccecum, the 

 dilated pharynx, the pharyngeal clefts, the hypophar- 

 yngeal gland, and the peripharyngeal bands, have been 

 inherited by all the Chordata (Vertebrata), and have 

 impressed themselves so firmly in their organization 

 that even the highest vertebrates still retain them, 

 either as vestiges or as organs which have been fitted 

 to new functions. I believe, however, that while they 

 were acquired before the tunicates diverged from the 

 chordate (vertebrate) stem, they were acquired by an 



^Studies from the Laboratory of the Johns Hopkins University, 1893, p. 175. 



