264 MOSTLY MAMMALS 
of a hippopotamus, we shall find that in each foot the four 
nearly equal-sized toes are severally supported by four 
complete and distinct bones, known in the fore-limb as 
the metacarpals and in the hind-limb as the metatarsals ; 
and it will be obvious that this is a much simpler or more 
generalised type of foot-structure than that which charac- 
terises the ruminants. If, again, we contrast the foot of a 
hippopotamus with that of a pig, we shall find that whereas 
in the latter the lateral pair of hoofs are considerably 
smaller than the middle pair and do not touch the ground 
when the animal is walking on a hard surface, in the 
former the two pairs are nearly equal in size and are all 
applied to the ground in walking. In this respect the 
hippopotamus is the most primitive of all the even-toed 
hoofed mammals that have survived to the present day, 
and is, therefore, a creature of special interest to the 
believer in evolution. It is, indeed, a member of the great 
group from which the ruminants are considered to have 
originated ; although, if the reader should be led from this 
statement to jump to the conclusion that a hippopotamus 
was in any sense an ancestor. of the giraffe, he would be 
led into a grievous error. As is the case with nearly all 
existing animals of a primitive type, the hippopotamus, in 
place of being an ancestral form, is a side branch from 
the original stock, which has developed certain specialised 
features not found in the latter. To show that this is the 
case, we have but to study the teeth of the various species 
of hippopotami, which are of such a nature as to show 
conclusively that those of the ruminants could not have 
been derived from them. 
In the group of animals last mentioned the molar-teeth 
have crescent-shaped columns on their grinding surfaces. 
Extinct animals show a complete passage from such teeth 
