308 LINKS BETWEEN EXISTING SPECIES. 



knowledge of any group increased, the separations between 

 the different species would become more and more unmistak- 

 able. On the contrary, however, it is a well-known fact that 

 the difficult genera become still more difficult as they are 

 more profoundly studied. If, indeed, we consider existing 

 forms only, no doubt the distinctions between the greater 

 number of species are well marked, nor does any one expect 

 to find a living series of links between them. The interme- 

 diate forms lived in tertiary and quaternary times. Thus 

 directly we commence to study the extinct forms, all the con- 

 venient lines of separation gradually thin out. For instance, 

 the larger species of mammalia are at present in most cases 

 well marked, but it becomes much more difficult satisfactorily 

 to distinguish them, from one another, when we consider fossil 

 specimens as well as recent ones. To take only two cases 

 from the group of quaternary mammalia, we have seen that, 

 according to Elitirneyer, the European and American bisons, 

 which are now quite distinct, are connected by the Bison 

 priscuSy while between our brown bear and the grizzly bear of 

 the Eocky Mountains a series of links has been discovered 

 among the abundant remains in our bone-caves. 



Great as is the interest attaching to the existence of man 

 at a period so much more ancient than that hitherto assigned 

 to him, there is something which, to many minds, will appear 

 even more fascinating, in the presence of such a fauna as that 

 which I have thus briefly indicated. For it must be regarded 

 as a well-ascertained fact, that, even during the human period, 

 the pleasant and sunny valleys of England and of France have 

 been inhabited by the gigantic Irish elk, two species of ele- 

 phant, and three of rhinoceros, together with the reindeer, a 

 large bear closely resembling the grizzly bear of the Rocky 

 Mountains, a bison scarcely distinguishable from that of the 

 American Prairies, the musk ox of Arctic America, the lem- 

 ming of the Siberian Steppes, the lion of the Tropics, the 



