OSTEOLOGY OE THE HYOPOTAMIM. 
73 
The general conclusion to be drawn from our survey of the fossil forms, is that 
they exhibit an extreme uniformity in the structure of their metapodium, and that these 
uniform and typical relations between the metapodium and the bones of the carpus and 
tarsus are remarkably constant, and, as regards the fossil forms, unpliant and rigid. Even 
with the utmost reduction of the metapodium in Anoplotlierium and Xiphodon , the 
typical relation between the two remaining metacarpals and metatarsals remains 
just the same as it probably was in their tetra- or pentadactyle ancestors ; so long as 
even a rudiment of a metacarpal or metatarsal remains, it holds just the same re- 
lation to the supporting bones as if it were complete. We meet with no distinct 
adaptation by means of which the median metacarpals and metatarsals which are left 
after the dropping off of the laterals, enter into a more complete articulation with 
the remaining bones of the carpus and tarsus. Considering, for instance, the two slender 
separate metacarpals and metatarsals of Xiphodon , we must confess that a foot so 
badly adapted for the use of a swift animal is rarely to be met with The middle 
digits, being unankylosed, are liable to be broken separately by a much less exertion 
of force than it would require to break the two coalesced middle digits of a Ruminant ; 
besides, seeing in what manner the whole weight of the body is transmitted to the two 
middle digits, we shall find that it is not effected in such a way as to ensure the 
most complete and stable equilibrium. To effect this we might expect that the 
proximal ends of the two remaining metacarpals (Plate XXXVII. fig. 3) would be 
enlarged to such a degree as to underlie the whole distal breadth of the carpus ; in this 
case the weight of the body would be transmitted much more equably and effectually to 
the two middle digits of the metacarpus. However, we see nothing of the kind : the 
transmission of the weight of the body is effected only by the two bones of the second 
row of the carpus ; and the two useless rudiments remaining on both sides, and occupying 
the whole trapezoid and a large facet of the unciform, diminish by so much the stability 
of the foot, in comparison with an arrangement in which the facets occupied uselessly 
by them should be taken by the functional middle digits. We meet with exactly the 
same relation in the pes ; so that it will be needless to recapitulate in reference to it all we 
have said in reference to the manus. The same may be, to a great extent, said of the 
Anoplotherium , though the digits which remain are so stout and short that the want of 
stability of the foot is not so clearly shown by this form as by its slender congener. 
On the whole we may, with sufficient probability, say that, while in these two genera 
the reduction of the manus and pes was going on and the lateral digits aborted, 
the remaining middle digits did not adapt themselves as fully as could be imagined* to 
altered circumstances of life and to a different distribution of weight of the body ; they 
remained too true to ancestral traditions ; there was no pliancy in their organization 
which, by adapting them more fully to altered conditions of life, would have enabled 
them to carry on successfully the struggle for existence with other competing genera. 
* And as we actually see in other genera which outlived Anoplotherium and Xiphodon. 
