THE LANCELET AND THE ASCIDIAN. 413 



on the hypothesis of this sedimentary stratification and of 

 this sequence, is the nature and origin of these rock-masses 

 intelligible. Since they are only conceivable and explicable 

 by these "geological hypotheses," these hypotheses are 

 universally accepted as " geological theories." 



On similar grounds, our phylogenetic hypotheses can 

 claim precisely the same force. In proposing them we 

 follow the same inductive and deductive methods, and with 

 the same approximate certaint}^, as are followed by geolo- 

 gists; because only with the aid of these phylogenetic 

 hypotheses is the nature and origin of Man and of other 

 organisms conceivable ; and because these hypotheses only 

 can satisfy our reason in its demand for causality, therefore 

 we hold these to be just ; therefore we claim for them the 

 rank of " biological theories." And, just as geological 

 hypotheses, which even in the beginning of the present 

 century were derided as speculative castles in the air, are 

 now universally accepted ; so, too, before the close of this 

 century will our phylogenetic hypotheses be received as 

 valid, although they are at present ridiculed- by the narrow- 

 minded majority of naturalists as "the dreams of the 

 physio-philosophers." It is true, our task, as we shall find, 

 is not so simple as that of the geologists. It surpasses the 

 latter in difiiculty and complexity in the same proportion as 

 the organization of Man is higher than the structure of the 

 rock.^^^ 



When we approach our task,^ we obtain very essential 

 aid by first closely studying the comparative germ-history 

 of two low animal forms. One of these is the Lancelet 

 (Araphioxus), and the other is the Sea-squirt (Ascidia) 

 (Plates X. and XI.). Both animals are extremely significant 



