The Zoology of North American Big Game 



were contained among these Condylarthra, as they 

 were named by Prof. Cope. 



Of course, these early mammals are known to us 

 only by their fossil and mostly fragmentary skele- 

 tons, but it may be said that at least in the ungulate 

 line, the successive geological periods show steady 

 structural progression in certain directions. Of 

 great importance are a decrease in the number of 

 functional digits; a gradual elevation of the heel, 

 so that their modern descendants walk on the tips 

 of their toes, instead of on the whole sole; a con- 

 stant tendency to the development of deeply 

 grooved and interlocked joints in place of shallow 

 bearing surfaces; and to a complex pattern of the 

 molar crowns instead of the simple type men- 

 tioned. To this may be added as the most im- 

 portant factor of all in survival, that these changes 

 have progressed together with an increase in the 

 size of the brain and in the convolutions of its 

 outer layer. 



The Condylarthra seem to have gone out of ex- 

 istence before the time of the middle Eocene, but 

 before this they had become separated into the two 

 great divisions of odd-toed and even-toed ungu- 

 lates, into which all truly hoofed beasts now living 

 fall. 



The first group (Perissodactyla) has always one 



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