6oo 



HISTORICAL GEOLOGY 



It will readily be seen that, if the weight of the body rested prin- 

 cipally upon the middle or third toes, and if the animal raised the heel 

 from the ground, the thumb or first finger would not ordinarily touch 

 the ground ; and if this habit of walking on the toes became better 

 developed in successive generations, not only the first toe but the 

 fifth as well might become of no use to the animal and might finally 

 atrophy. A continuation of the process, accompanied by a length- 

 ening of the foot, would result in the dropping of the second and 



fourth toes and in the formation 

 of the highly specialized, one-toed 

 foot of the horse (p. 608). 



If the weight, instead of being 

 directly on the middle toe, was 

 between the third and fourth toes, 

 a more digitigrade habit (walking 

 on the toes) would result in the 

 reduction and later dropping of 

 the thumb or first finger, leaving 

 a four-toed foot. By the further 

 reduction in size of the side toes, 

 a foot like that of a pig, with two 

 strong toes and two small ones, 

 would result. When these side 

 toes disappeared, the animal had 

 but two toes on each foot, like the camel and sheep. Judging from 

 the abundance of the even-toed ungulates (artiodactyls), it seems 

 that, as a whole, this type of foot has proved to be the best. These 

 modifications in foot structure apparently were the result of a 

 change from the forest conditions of the Eocene, where soft ground 

 and succulent vegetation were the rule, to the plains vegetation 

 (p. 630) of the later times, with their siliceous grasses where a short, 

 spreading foot would not give the animal the speed necessary to 

 move long distances in a short time, for food and water (p. 610). 



Factors in the Evolution of Mammals 



In the course of the history of the mammals to be studied it will be 

 found that, beginning with some such ancestor as Phenacodus, which 

 is full of mechanical imperfections, the skeletons were modified chiefly 

 in four particulars, each of which was of more or less vital impor- 

 tance to the various races affected. 



ABC 



Fig. 541. — Evolution of the foot of 

 odd-toed mammals (perissidactyls), illus- 

 trated by existing families: A, tapir; 

 B, rhinoceros; C, horse. (After Bed- 

 dard.) 



