THE OVARIAN EGG. yal 
No sooner were these facts established, than 
the relation between the extinct world and the 
world of to-day became the subject of extensive 
researches and comparisons; innumerable theo- 
ries were started to account for the differences, 
and to determine the periods and manner of the 
change ; and the science of Paleontology became 
one of the most important departments of inves- 
tigation in modern times. It is not my intention 
to enter now at any length upon the subject of 
geological succession, though I hope to return to 
it hereafter in a series of papers upon that and 
kindred topics; but I allude to it here, before 
presenting some views upon the maintenance of 
organic types as they exist in our own period, for 
the following reason. Since it has been shown 
that from the beginning of Creation till the pres- 
ent time the physical history of the world has 
been divided into a succession of distinct periods, 
each one accompanied by its characteristic ani- 
mals and plants, so that our own epoch is only 
the closing one in a long procession of ages, 
naturalists have been constantly striving to find 
the connecting link between them all, and to 
prove that each such creation has been a normal 
and natural growth out of the preceding one. 
With this aim they have tried to adapt the phe- 
nomena of reproduction among animals to the 
problem of creation, and to make the beginning 
