172 THE POPULAR SCIENCE MONTHLY. 



that, throughout the prodigious duration of time registered by the fos- 

 siliferous rocks, the living population of the earth had undergone con- 

 tinual changes, not merely by the extinction of a certain number of 

 the species which at first existed, but by the continual generation of 

 new species, and the no less constant extinction of old ones. 



Thus, the broad outlines of paleontology, in so far as it is the com- 

 mon property of both the geologist and the biologist, were marked 

 out at the close of the last century. In tracing its subsequent progress 

 I must confine myself to the province of biology, and indeed to the 

 influence of paleontology upon zoological morphology. And I accept 

 this limitation the more willingly as the no less important topic of the 

 bearing of geology and of paleontology upon distribution has been 

 luminously treated in the address of the President of the Geographical 

 Section. 



The succession of the species of animals and plants in time being 

 established, the first question which the zoologist or the botanist had to 

 ask himself was, " What is the relation of these successive species one 

 to another ? " And it is a curious circumstance that the most impor- 

 tant event in the history of paleontology which immediately succeeded 

 AVilliam Smith's generalization was a discovery which, could it have 

 been rightly appreciated at the time, would have gone far toward 

 suggesting the answer, which was in fact delayed for more than half 

 a century. I refer to Cuvier's investigation of the mammalian fossils 

 yielded by the quarries in the older Tertiary rocks of Montmartre, 

 among the chief results of which was the bringing to light of two 

 genera of extinct hoofed quadrupeds, the Anoplotherium and the 

 Palceotherium. The rich materials at Cuvier's disposition enabled 

 him to obtain a full knowledge of the osteology and of the dentition 

 of these two forms, and consequently to compare their structure crit- 

 ically with that of existing hoofed animals. The effect of this com- 

 parison was to prove that the Anoplotherium, though it presented 

 many points of resemblance with the pigs on the one hand, and with 

 the ruminants on the other, differed from both to such an extent that 

 it could find a place in neither group. In fact, it held, in some re- 

 spects, an intermediate position, tending to bridge over the interval 

 between these two groups, which in the existing fauna are so distinct. 

 In the same way, the Palceotherium tended to connect forms so differ- 

 ent as the tapir, the rhinoceros, and the horse. Subsequent investiga- 

 tions have brought to light a variety of facts of the same order, the 

 most curious and striking of which are those which prove the exist- 

 ence, in the Mesozoic epoch, of a series of forms intermediate between 

 birds and reptiles two classes of vertebrate animals which at present 

 appear to be more widely separated than any others. Yet the interval 

 between them is completely filled, in the mesozoic fauna, by birds 

 which have reptilian characters on the one side, and reptiles which 

 have ornithic characters on the other. So, again, while the group of 



