THE EVOLUTION 

 OF THE HORSE 



While some animals, as shown in 

 the frontispiece of this? issue, have 

 undergone practically no change in 

 a hundred million years or more, 

 other whole groujjs, such as the 

 mammals, have in only a twentieth 

 or a thirtieth of that time under- 

 gone an amount of change that is, 

 by comparison, cxtermely large. No 

 case of this sort is better known 

 than the horse, who has altered in 

 the last three or four million years 

 in almost every way that a mam- 

 mal can alter. It further alJl3ear^ 

 that development was not at all uni- 

 form during this time; there would 

 be little change for a long })eriod, 

 and then rapid modification would 

 take place. 



The only other mammal well 

 enough known to offer a parallel to 

 the horse, in the extent of change 

 that has'taken jilace in a short time, 

 is man himself, whose ancestor in the 

 early Eocene period was probably a 

 little tree-dwelling animal of the 

 tropics which a casual observer 

 might mistake for a squirrel. 



The horse of the early Eocene is 

 shown in the small insert below, 

 from fossil remains found in New 

 Mexico, and lately restored Ijy J. W. 

 Gidley, assistant curator of fossil 

 mammals at the U. S. National 

 Museum. This Eohippus was about 

 1-4 inches high at the shoulder and 

 had four well-formed toes, with a 

 vestige of a fifth toe. He probably 

 lived around the margins of lakes 

 and pastured on grass. 



A Belgian mare, photographed 

 on the same scale, is shown for 

 comparison. The most remarkable- 

 change in the course of evolution 

 has been in size, but the loss of all 

 the toes except one is also note- 

 worthy, as well as a great progres- 

 sive lengthening of the cheek-teeth. 

 It is true, of course, that part of 

 the evolution of the horse has been 

 due, in the last few thousand years, 

 to artificial selection by man. This 

 has Ijrought about much of the in- 

 crease in size. But the difference 

 between Eohijjpus and modern wild 

 horses is striking enough. 



It is necessary to suppose that 

 the germ-plasm of some species is 

 much more "])lastic" and variable 

 than that of others. This is especi- 

 ally true of man and the horse. 

 Wiien man's ancest(jrs came down 

 out of the trees, and when Eohij)- 

 pus emerged from the swamjjs and 

 began to roam over the prairies, 

 rai)i(l change was possible. (Fig. 7). 



