PalcEontology. 193 



ifitructure in any of the existing hoofed animals, we 

 find that the bony processes called zygapophyses, 

 which belong to each of the constituent vertebrae, 

 are so arranged that the anterior pair belonging to 

 each vertebra interlocks with the posterior pair be- 

 longing to the next vertebra. In this way the whole 

 series of vertebrae are connected together in the form 

 of a chain, which, while admitting of considerable 

 movement laterally, is everywhere guarded against 

 dislocation. But if we examine the skeletons of any 

 ungulates from the lower Eocene deposits, we find 

 that in no case is there any such arrangement to 

 secure interlocking. In all the hoofed mammals of 

 this period the zygapophyses are flat. Now, from 

 this flat condition to the present condition of full 

 interlocking we obtain a complete series of connecting 

 links. In the middle Miocene period we find a group 

 of hoofed animals in which the articulation begins 

 by a slight rounding of the previously flat surfaces : 

 later on this rounding progressively increases, until 

 eventually we get the complete interlocking of the 

 present time. 



As regards teeth, and still confining attention to 

 the hoofed mammals, we find that low down in the 

 geological series the teeth present on their grinding 

 surfaces only three simple tubercles. Later on a 

 fourth tubercle is added, and later still there is de- 

 veloped that complicated system of ridges. and furrows 

 which is characteristic of these teeth at the present 

 time, and which was produced by manifold and 

 various involutions of the three or four simple tuber- 

 cles of Eocene and lower Miocene times. In other 

 words, the principle of gradual improvement in the 



