34 



CAUSES OF VARIATION 



life the Mississippi valley was just emerging from the Gulf of 

 Mexico, and the plains of western Europe and Asia were low 

 and wet. The climate was moist and tropical, stimulating dense 

 and luxuriant growth of giant vegetation even as far north as 

 Greenland. With the Tertiary came a general elevation, usher- 

 ing in a comparatively cold, dry climate, favorable to grasses 

 and the harder vegetation generally. With this came grassy 

 plains and the evolution of races with good teeth and excellent 

 feet and legs, fitting them to a life in the open. 



With these profound changes in nature other forms under- 

 went a development similar to that of the ancestors and other 

 relatives of the horse. Many of these, as our cattle, sheep, swine, 

 etc., developed a two-toed foot, and some, as the rhinoceros, 

 stopped at the three-toed stage, but none of them became so 

 highly specialized as the horse. 



Here was a great line of descent, continuing almost for ages, 

 and terminating in many highly specialized species that are still 

 flexible. But it gave rise on the way down (or up) to many known, 

 and doubtless to many unknown, branches that became extinct 

 through some general disaster, or, more likely, because of their 

 inherent inability to develop all the characteristics necessary to 

 meet changing conditions. For example, as the teeth developed 

 into molars fitted for grinding the ever-hardening forage, some 

 species secreted cement in the valleys thus supporting the hard 

 and grinding ridges; others did not, and it is significant that in 

 the latter case no species endured. 1 The elephant alone, of his 

 kind, has persisted to the present, and if this is because of his 

 teeth, and in spite of body and feet, which are ill adapted to 

 modern conditions, it serves to show on how slender a thread 

 the life of a species often hangs. 



Present existing land species are to be regarded as representing 

 lines of descent naturally endowed with an unusually high degree 

 of flexibility ; all the more stable and less adaptable forms having 

 perished off the earth in the long struggle to keep up with the 



1 It is worthy of remark that the central plains of South America seem to 

 have developed a horse-like animal (Litopterna), losing its lateral toes and develop- 

 ing the hinge joint and lengthened limb ; but it never developed cement in its 

 grinders, which remained inferior, and we are not surprised that its line became 

 extinct. 



