z 9 o THE POPULAR SCIENCE MONTELY. 



of bony matter which is called cement. Consequently the surface of 

 the tooth is composed of very uneven materials of the hard mass of 

 the tooth, which is called dentine, then a very much harder enamel, 

 and a softer cement between, the practical effect of which is the same 

 as the lamination of the millstone. In consequence of the lamination 

 of the millstone the ridges wear less swiftly than 1^ie intermediate 

 substance, and therefore the surface always keeps rough and exerts 

 a crushing effect upon the grain. The same is true of the horse's 

 tooth, and consequently the grinding of the teeth one against the 

 other, instead of flattening the surface of the teeth, tends to keep 

 them always irregular, and that has a very great influence upon the 

 rapid mastication of the hard grain or the hay upon which the 

 horse subsists. 



I think that will suffice as a brief indication of some of the most 

 important peculiarities and characteristics of the horse. If the hy- 

 pothesis of evolution is true, what ought to happen when we investi- 

 gate the history of this animal? We know that the mammalian type, 

 as a whole that mammalian animals are characterized by the pos- 

 session of a perfectly distinct radius and ulna, two separate and dis- 

 tinct movable bones. We know, further, that mammals in general 

 possess five toes, often unequal, but still as completely developed as 

 the five digits of my hand. We know further that the general type 

 of mammal possesses in the leg, not only a complete tibia, but a com- 

 plete fibula a complete, distinct, separable bone. Moreover, in the 

 hind-foot we find, in animals in general, five distinct toes, just as we 

 do iu the fore-foot. Hence it follows a differentiated animal like the 

 horse must have proceeded by way of evolution or gradual modifica- 

 tion from a form possessing all the characteristics we find in mam- 

 mals in general. If that be true, it follows that if there be anywhere 

 preserved in the series of rocks a complete history of the horse, that 

 is to say of the various stages through which he has passed, those 

 stages ought gradually to lead us back to some sort of animal which 

 possessed a radius, and an ulna, and distinct complete tibia and fibula, 

 and in which there were five toes upon the fore-limb, no less than 

 upon the hind-limb. Moreover, in the average general mammalian 

 type, the higher mammalian, we find, as a constant rule, an approxi- 

 mation to the number of forty-four complete teeth, of which twelve 

 are cutting teeth, four are canine, and the others are grinders. In 

 unmodified mammals we find the incisors have no pit, and that the 

 grinding teeth, as a rule, increase in size from that which lies in front 

 toward those which lie in the middle or at the hinder part of the series. 

 Consequently, if the theory of evolution be correct, if that hypothe- 

 sis of the origin of living things have a foundation, we ought to find 

 in the series the forms which have preceded the horse, animals in 

 which the mark upon the incisor gradually more and more disappears, 

 animals in which the canine teeth are present in both sexes, and ani- 



