2o6 ANIMAL LIFE PAST AND PRESENT. 



Cattle ; Tapirs, Ehinoceroses, and Horses ; and, lastly, 

 Elephants. The animals of the first group are all 

 related to one another by the structure of the feet, in 

 which the two middle toes are symmetrical to one 

 another; and we can trace in this, as well as in the 

 two following groups, a gradual increase in the com- 

 plexity of the teeth, as we proceed from what naturalists 

 term the generalised to the specialised types; the 

 latter being, it need hardly be mentioned, mainly 

 characteristic of the present day and the latest geo- 

 logical period, while the former are mostly of earlier 

 origin. 



The simplest form of cheek-tooth in this group is 

 found among certain extinct animals closely related to 

 the modern Pigs, three of these teeth being repre- 

 sented in Fig. 65. It will there be seen that the crown 

 of each of these teeth carries four comparatively low 

 and simple cone-like prominences ; the number of 

 these cones being one added to the primitive type 

 of triangular tooth mentioned above. These cones, it 

 will be observed, are separated from one another by a 

 + -shaped shallow depression or valley ; and when their 

 summits become somewhat worn down by use (as in 

 the tooth on the left of the figure), it will be seen that 

 nearly circular islands of dentine are exposed. The 

 figure also shows that the last tooth is rather larger 

 than either of the others; this being still more 

 markedly the case with the corresponding tooth in 

 the lower jaw. In the modern Pigs this last lower 

 tooth (Fig. 66) becomes still larger ; while, as may be 



