THE OVARII tfGG. 271 



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 



