I26o CLASS MAMMALIA. 



regarded merely as the commencement of the epoch in which we 

 are now living, the greater number of the Mammals of Europe 

 belong to existing genera, and a considerable proportion of these to 

 living species. The fauna of Europe in the early part of this period 

 included, however, a large number of Mammals belonging to genera 

 or species now confined to the warmer regions of the globe, such 

 as Rhinoceroses, Hippopotami, Elephants, Lions, Hyaenas, &c. ; 

 and it was not till after the great cold of the glacial period that 

 these generic types were finally swept away from the European 

 area. In many other parts of the World the Pleistocene period 

 was equally prolific in large forms of Mammalian life, more or less 

 closely allied to those now inhabiting the same areas, of which we 

 have remarkable instances in the extinct Edentates of South America 

 and the Marsupials of Australia, many of which vastly exceeded in 

 size their living relatives. Science has, indeed, yet to account satis- 

 factorily for the disappearance of this exuberant life, and the conse- 

 quently impoverished fauna among which we now dwell. 



In regard to the Mammals of the Eocene, we have already re- 

 marked that they are frequently of a more generalised type than 

 those now existing, and in many groups a well-marked progressive 

 specialisation can be traced as we approach the existing epoch. 

 Thus many of the Eocene Mammals possess the full Eutherian 

 complement of forty-four teeth, which in the Ungulates were more 

 uniform in size, less conspicuously differentiated into groups, and 

 more approximated to one another than in recent forms. In the 

 older Ungulates, moreover, the crowns of the cheek-teeth were re- 

 latively short ; and we can trace a gradual increase in the height of 

 the crowns as we advance in time, this increase affording a greater 

 capacity to withstand wear, and thus indicating a greater length of 

 life in the individual. In other instances we may observe a gradual 

 reduction in the lateral digits of the typical pentedactylate limb, 

 accompanied by a progressive elongation and strengthening of one 

 or more of the remaining digits. Again, it has been shown that 

 there has been a gradual increase in the relative size and the com- 

 plexity of the brain as we approach the present day. Thus the 

 Eocene Mammals, as a rule, had very small brains, in which the 

 hemispheres left the cerebellum nearly uncovered, and were them- 

 selves nearly smooth, or but slightly convoluted ; while in the higher 

 forms the hemispheres spread backwards over the cerebellum, and 

 are often marked by most complex convolutions. In some Eocene 

 Ungulates the brain was so small that it could pass through the 

 neural canal of the lumbar vertebrae. 



Finally, it should be observed that we may trace a gradual evolu- 

 tion of local faunas. In the Eocene of any given region we find 

 the Mammals differing widely in generic types from those now in- 



