708 GENETICS AND EVOLUTION 



EOCENE OLIGOCENE MIOCENE 



PLIOCENE PLEISTOCENE-RECENT 



Eohippas Miohippus Mzrychippus Pliohippus Ecfuus 



Figure 34.3. Stages in the evolution of the horse, illustrating (top) the changes 

 in size and shape (the numbers indicate the shoulder height in inches), (second row) 

 the bones of the fore and hind feet, (third row) the skull, and (bottom) the grinding 

 surfaces of the second upper molar tooth. Eohippus is a synonym of Hyracotherium. 



instances often quoted as examples of orthogenesis are not truly evolu- 

 tion in a straight line. The horse is often said to have evolved in a 

 straight line from the primitive Hyracotherium (a small animal, the 

 size of a fox, with four toes on the front feet and three toes on the hind 

 feet) to the modern Equus, but the complete fossil record shows that 

 there were many side branches in horse evolution (Fig. 34.3). The evo- 

 lution of the present-day horse is not at all the simple progression along 

 a single, straight line of evolution that it was once thought to be. The 

 evolution of the horse was said to show the following "trends": an 

 increase in size, a lengthening of the legs, enlargement of the third 

 digit and reduction of the others, an increase in the size of the molar 

 teeth and in the complexity of the patterns of ridges on their crowns, 

 and increases in the size of the lower jaw and the skull. More recent 

 work has shown that there are so many exceptions to each of these that 

 the concept of a straight-line evolution of the horse has been abandoned. 

 The term orthogenesis is sometimes applied to the evolutionary over- 

 development of some characteristic. The classic example of this is the 

 development of the antlers of the extinct Irish deer. In successive genera- 

 tions the antlers became larger, and although this may have been of 



