9 2 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



longitudinal folds. This hypothesis is further sus- 

 tained by the acanthodean genus Climatius, where a 

 series of spines intervenes between the paired fins in 



line with them. From what the Acanthodeans can 

 have been descended is at present conjectural. They 

 trace their ancestry ultimately to Branchiostoma (Am- 

 phioxus) through forms not yet discovered. This ge- 



