Horse evolution 



656 ORGANIC EVOLUTION 



the animals were fertile. These and many the resulting organisms will then be able to 



other slight differences act as barriers to change only in certain ways. This would 



keep the two species distinct and separate, have the effect of guiding any possible 



Many kinds of gene changes occur, from mutations in a general direction so that 

 slight modifications in the gene itself to an evolution could proceed in that direction 

 alteration in the arrangement of the genes and no other. While evolution may have 

 on the chromosomes, any of which may been random at first, it has since become 

 make fertility impossible. Under laboratory oriented or directed so that today it is oc- 

 conditions these changes sometimes occur curring along specific lines. This idea is 

 spontaneously in a species that in all other often referred to as orthogenesis or straight- 

 respects is like the parent form. Such ge- line evolution. While, in general, it does 

 netic changes are as effective in isolating occur and evolution is not totally random, 

 animals as any physical barrier could be. it is not as directional as was once thought. 

 Many biologists agree that interfertility is One of the classic evidences for straight- 

 the acid test of a species. No matter what line evolution is that of the horse, 

 other characteristics two species may have 

 in common, they are distinct species if they 

 lack interfertility. Tlie problem of establish- The modern horse has had an interesting 

 ing species on such a basis is almost hope- evolution and one that can be followed 

 less at the present time because of our lack better than most because of the abundance 

 of information concerning the genetic con- of fossils left from the Eocene to the present 

 stitution of all but a few species of animals, and because most of them occur in North 

 These are the animals extensively used in America where much digging has been go- 

 the laboratory, such as Drosophila, mice, ing on during the past 200 years, 

 rats, and paramecium. Eohippus, the earliest known ancestor of 



the horse, and perhaps the tapirs and rhi- 



DIRECTION OF EVOLUTION noceroses as well, occurred in early Eocene 



times and was anatomically quite remote 



At the beginning of life there must have from Equus, our modern horse (Fig. 26-4). 

 first existed only one species. After a time It was about the size of a jackrabbit and 

 this must have varied as a result of muta- possessed functional toes on each append- 

 tions, thus giving natural selection an op- age, four in front and three behind. There 

 portimity to initiate evolution. It is conceiv- were rudiments of others which, when ac- 

 able that evolution might have proceeded counted for, demonstrated that the animal 

 in any or all directions, and perhaps it did had descended from an ancestor with five 

 at first, but after a time it seems to have digits, the usual number among verte- 

 headed in a certain rather general but defi- brates. In the forefeet only the inside or 

 nite direction, which has continued more "thumb" was rudimentary while in the hind 

 or less uninterrupted ever since. This is as feet both the thumb and tlie "little finger" 

 one would expect, because genes, being were missing but could be seen as function- 

 chemical entities, would be apt to change in less splints along the next foot bone. The 

 fairly similar ways. Because of their struc- teeth of Eohippus were also quite different 

 tural nature it would seem impossible for from those of the modern horse. They had 

 them to change in any or all directions, short crowns and long roots nicely adapted 

 Similar mutations do appear repeatedly, for browsing in and around the edges of 

 as a matter of fact, indicating that the same forests. The face was short, the eyes placed 

 gene changes again and again. Since muta- well over the teeth. These tiny mammals 

 tions can occur only within certain limits, apparently lived in seclusion, feeding on 



