10 



illustration of the general fact that the vestigial organs are apt to cause trouble. 

 Like the vermiform appendix, this is a condition of unstable equilibrium. 



We observe that the evolution of the teeth of the horse, terminates in the 

 elaborate crown of the very complex teeth of the modern horse. The dental 

 battery of the modern horse, explains the elongation of the face ; at the time when 

 the full set of grinders have reached their maximum of development, before the 

 procees of wearing off these crowns has been proceeding for any length of time, 

 in other words, while these permanent grinders are in a perfect condition, a great 

 deal of space is needed for them. This is provided for by the expansion of the 

 face above and of the jaw below, and accounts for the great development of the 

 skull in front of the orbits or eye-sockets in course of the evolution of the horse. 



This change extends the age of the horse. The little horse (Eohippus) 

 could not have lived much more than eight or ten years, about the age of the 

 whippet. We have just succeeded in securing for our collection a specimen of 

 thej iorse whk h has reached anage^^of over 31 years; in this animal the teeth 

 are worn do wn so~that "Tfie^TaTe barajly half an inch in length; whereas, when 

 in perfect condition the teeth are «« inches in length. A horse dies partly 

 from mal-nutrition, owing to the giving out of its very efficient dental bat- 

 tery. The pocketed cropping teeth of the horse are also a process of long evolu- 

 tion, the aged teeth becoming perfectly smooth and less effective. Fig. 20. 



Distribution and Migration of the Horse. 



A map of the United States is interesting simply as showing the very wide 

 distribution of the horse on the American continent, and the fact that before 

 the extinction of the horse in America we have evidence, although we have not 

 been able to pursue this evidence to a point of obtaining complete specimens, 

 that the most superb natural breeds of horse, varying from a size far 

 smaller than the smallest modern Shetland, to a size far exceeding that of the 

 largest modern Percheron, were developed by natural processes of breeding and 

 selection on this continent. It seems like a tragedy that at this climax of evo- 

 lution, which nature had been slowly shaping through such a long period of 

 time, horse life in America should have come to a complete and absolute close 

 with the appearance of the great ice cap from the north and with the onset of 

 the so-called Glacial Period. At the beginning of this period America was 

 thickly populated with horses, extending down into South America. At the 

 end of that period not a single horse remained either in North or South America. 



The map represents the condition just prior to the glacial period, in which 

 we find horses of varied type in both North and South America and as far north 

 as Escholz Bay on the Arctic Ocean. 



After the glacial period, these animals are confined to a region extending 

 from Central Asia over North Africa, down to the southern extremity of South 

 Africa, the entire race having been swept off the Americas. 



I am not satisfied that this widespread extinction was entirely caused by 

 the ice age. It seems that we must bring in some other possible agency, and I 

 am at present engaged in studying the epidemic diseases of horses in various 



