242 LOUIS AGASSIZ. 



given him the key to past creations, how the 

 complete skeleton of the living fishes has ex- 

 plained the scattered fragments of the ancient 

 ones, especially those of which the soft carti- 

 laginous structure was liable to decay, he pre- 

 sents two modes of studying the type as a 

 whole ; either in its comparative anatomy, in- 

 cluding in the comparison the whole history 

 of the type, fossil and living, or in its com- 

 parative embryology. " The results," he adds, 

 " of these two methods of study complete and 

 control each other." In all his subsequent 

 researches indeed, the history of the individ- 

 ual in its successive phases went hand in hand 

 with the history of the type. He constantly 

 tested his zoological results by his embryolog- 

 ical investigations. 



After a careful description of the dorsal 

 chord in its embryological development, he 

 shows that a certain parallelism exists between 

 the comparative degrees of development of 

 the vertebral column in the different groups 

 of fishes, and the phases of its embryonic de- 

 velopment in the higher fishes. Farther on 

 he shows a like coincidence between the devel- 

 opment of the system of fins in the different 

 groups of fishes, and the gradual growth and 

 differentiation of the fins in the embryo of the 



