114 THE THEORY OF EVOLUTION 



tion, with diversification and adaptation to different 

 climates, food-plants and habits of life. 



There remains one family of mammals with which 

 it is necessary to deal and that is the camel tribe. 

 This family has two well-defined subdivisions, the 

 camels of the Old World and the llamas, guanacos, 

 etc., of South America. For a very long time, the 

 family was entirely confined to North America and 

 did not reach its present homes until the Pliocene 

 epoch of the Tertiary period. The skeleton of a 

 Patagonian Guanaco may be taken as the starting- 

 point of our inquiry. In this animal the third incisor 

 and the canine are retained in the upper jaw, all the 

 incisors and the canine in the lower. The anterior 

 two grinding teeth have been lost and the others are 

 moderately high-crowned. The skull is broad and 

 capacious behind, narrow and tapering in front. 

 The neck is long and its vertebrae very curiously 

 modified. The limbs are long and slender and have 

 undergone nearly the same modifications as in the 

 horses; the ulna is greatly reduced, interrupted in 

 the middle and its separated portions are fused with 

 the radius. In the hind leg the shaft of the fibula 

 has been completely suppressed; the upper end fuses 

 with the tibia, while the lower remains as a small 

 separate bone, wedged in between the tibia and the 

 heel-bone. The feet are very long and slender, with 

 two toes in each; the long bones of the foot are co- 

 ossified to form a "cannon-bone," the very young 

 skeleton showing that this co-ossification does ac- 



