296 Agassiz— Paleontological and Hmbryological Development. 
haps cannot be better expressed than by comparing the fauna 
of any period as a whole with that of following epochs ;—a 
zo0logical system of the Jura, for instance, compared with one 
made up for the Cretaceous; next, one for the Tertiary, com- 
pared with the fauna of the present day. In no case could we 
find any class of the animal kingdom bearing the same defini- 
tions or characterized in the same manner. But apply to this 
comparison the data obtained from the embryological develop- 
ment of our present fauna, and what a flood of light is thrown 
upon the meaning of the succession of these apparently dis- 
connected animal kingdoms, belonging to different geological 
periods, especially in connection 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 na- 
ture of the material and of my own familiarity with their de- 
velopment 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 knowl- 
edge 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 representatives 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 Juras- 
sic period, to call your attention to the paleontological history 
of the group, and to compare the succession of its members 
with the embryological development of the types now living in 
our seas. mple material for making this comparison is 
fortunately at hand; it is material of a peculiar kind, not 
easily obtained, and which thus far has not greatly attracted 
the attention of zodlogists. 
Interesting and important as are the earliest stages of embry- 
onic development in the different classes of the animal king- 
dom, as bearing upon the history of the first appearance of 
