Agassiz— Paleontological and Embryological Development. 387 
which have continued through two or three great periods, we 
must likewise accord to their latest representatives a direct de- 
scent from theolder. The very fact that the ocean basins date 
ack to the earliest geological periods, and have afforded to 
the marine animals the conditions most favorable to an unbroken 
ancestors of the Cidaris of to-day. The Saleniz of the lower 
Chalk are those of the Salenia of to-day. Acrosalenia extends 
from the Lias to the lower Cretaceous, with a number of re- 
cent genera, which begin at the Eocene. The Pygaster of to- 
day dates back to the Lias; Echinocyamus and Fibularia com- 
mence with the Chalk. Pyrina extends from the lower Jura 
through the Eocene. The Echinobrissus of to-day dates back 
to the Jura. Holaster lived from the lower Chalk to the Mio- 
cene, and the Hemiaster of to-day cannot be distinguished 
from the Hemiaster of the lower Cretaceous. 
better acknowledge our 
inability to go beyond a certain point; anything beyond the 
general parallelism I have attempted to trace, which in no way 
sewalidiates the other proposition, we must recognize as hope- 
ess. 
But in spite of the limits which have been assigned to this 
general parallelism, it still remains an all-essential factor in 
elucidating the history of paleontological development, and its 
importance has but recently been fully appreciated. For, 
while the fossil remains may give us a strong presumptive evi- 
dence of the gradual passage of one type to another, we can 
only imagine this modification to take place by a process 
similar to that which brings about the modifications due to 
different stages of growth,—the former taking place in bh 
may practically be considered nfinite time when com 
to the short life history which has given us as it were a résumé 
of the paleontological development. e well pause to 
