1 18 



UNDKRIIILI. 



Figuratively speaking-, our highly individualized animals 

 of to-day represent the topmost twigs of a tree. In the pre- 

 ceding branches there is a tendency to combine these charac- 

 teristics in more comprehensive types, and we find ancestral 

 forms the more generalized as we pass downward towards the 

 trunk. At the point where we meet the first Ungulate we find 

 evidences that a branching has already taken place into odd- 

 hoofed (Perissodactyla), where the middle toe is the centre of 

 support, and into pair-hoofed ( Artiodactyla ), where the sup- 

 port is distributed between the middle and fourth. The pale- 

 ontologist can by most probable outlines trace these two 

 groups to the present, where they reach their highest expres- 

 sion in the liquidcc and Bovidcc. Primitive Ungulates first 

 appear in the lowest Eocene formations of the western lake 

 basins. These belong to a single genus, dnyphodon, having 

 five tt)es in front and behind, with the third or middle toe 

 decidedly the best developed, thus showing the odd-hoofed 

 tendency. But this largest mammal of the Lower Eocene 

 sheds very little light upon the five-toed beginnings of our 

 little four-toed horses, and it leaves a gap yet to be filled to 

 the first incomings of the Hoofed Animals. Indeed the obscu- 

 rity is general as to the source of the mammalian assemblage 

 that here makes its appearance. Though it has left us a 

 record through the Tertiary Age that is almost complete, 

 below the Tertiary it seems lost, only a few diminutive mam- 

 mals of very low type having been yielded thus far by the 

 Cretaceous. The case is thus referred to by Le Conte : "It is 

 impossible to explain this unless we admit times of rapid evo- 

 lution. But even this is not sufficient. We must suppose, 

 also, that these new types appeared here in America by migra- 

 tion about the end of the Cretaceous from some other country, 

 where we hope yet to find the intermediate links." 



Over forty years ago a skull was found in the Lower 

 Eocene of England belonging to an animal which was at that 

 time named the Hyracotherium by Pn^fessor Owen and which 

 has since been recognized by paleontologists as representing 



