350 Prof. A. Agassiz on Palceontological 



general way, even for a single class, the agreement known to 

 exist in certain groups between their embryonic development 

 and their palrcontological history. It is hinted at in the 

 succession of animal life of any period we may take up, and 

 perhaps cannot be better expressed than by comparing the 

 fauna of any period as a whole with that of following- 

 epochs — a zoological system of the Jura, for instance, com- 

 pared with one made up for the Cretaceous ; next, one for the 

 Tertiary compared with the fauna of the present day. In 

 no case could we find any class of the animal kingdom 

 bearing the same definitions or characterized in the same 

 manner. But apply to this comparison the data obtained 

 from the embryological development of our present fauna, 

 and what a flood of light is thrown upon the meaning of the 

 succession of these apparently disconnected animal kingdoms, 

 belonging to different geological periods, especially in con- 

 nexion with the study of the few ancient types which have 

 survived to the present day from the earliest times in the 

 history of our earth ! 



Although there is hardly a class of the animal kingdom 

 in which some most interesting parallelism could not be 

 drawn, and while the material for an examination of this 

 parallelism is partially available for the Fishes, Mollusks, 

 Crustacea, Corals, and Crinoids, yet for the illustration and 

 critical examination of this parallelism I have been led to 

 choose to-day a very limited group, that of Sea-urchins, 

 both on account of the nature of the material, and of my 

 own familiarity with their development and with the living 

 and extinct species of Echini. The number of living species 

 is not very great (less than three hundred) , and the number 

 of fossil species thus far known is not, according to Zittel, 

 more than about two thousand. It is therefore possible for a 

 specialist to know of his own knowledge the greater part of 

 the species of the group. It has been my good fortune to 

 examine all but a few of the species now known to exist, and 

 the collections to which I have had access contain representa- 

 tives of the majority of the fossil species. Sea-urchins are 

 found in the oldest fossiliferous rocks ; they have continued 

 to exist without interruption in all the strata up to the present 

 time. While it is true that our knowledge of the Sea-urchins 

 occurring before the Jurassic period is not very satisfactory, 

 it is yet complete enough for the purposes of the present 

 essay, as it will enable me, starting from the Jurassic period, 

 to call your attention to the palaontological history of the 

 group, and to compare the succession of its members with the 

 embryological development of the types now living in our 



