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The 'Evolution of the Horse. 
metrical in itself, the free border of the last bone (which 
supports the hoof) being evenly rounded. This may be the 
only toe sufficiently large to be of any use to the animal, or the 
second and fourth may be equally developed on each side of it. 
In tapirs, and in many extinct forms, the fifth toe also is present 
on the fore-foot ; but this does not interfere with the symmetrical 
arrangement of the remainder of the foot around the median 
line of the third, or middle digit. A first toe or thumb (pollex), 
completing the typical five, has only been found in some ex- 
tremely ancient and primitive forms. 
A 3 
Fig. 1. — A. Diagrammatic representation of the fore-foot of an odd-toed or 
Perissodactyle animal, b. Of an even-toed or Artiodactylc. C. Carpus 
or wrist. M. Metacarpus. The toes are numbered as generally counted in 
order from the inner to the outer side of the foot. The shaded parts of A 
are those that are present in the Horse ; in B, those that are present in the O.v. 
It is only by studying the fundamental type of organisation 
common to all the members of a group, which underlies the 
various external or superficial modifications by which it becomes 
adapted to the different surrounding conditions under which it 
has to carry on its existence, that the true relationship of 
animals can be determined. In this way it can be clearly 
demonstrated that the pig, the deer, the ox, sheep, goat, antelope, 
and camel, including even such extreme forms as the giraffe 
and the hippopotamus, are formed on one common plan — the 
