Paleontology. 183 
whether an animal has an even or an odd number 
of tocs may seem a curiously artificial d'stinction 
on which to found so important a classification 
of the mammalian group: But if we look at the 
matter from a less empirical and more intelligent 
point of view, we shall see that the alternative of 
having an even or an odd number of toes carries with 
it alternative consequences of a practically important 
kind to any animal of the digitigrade type. For 
suppose an aboriginal five-toed animal, walking on 
the ends of its five toes, to be called upon to resign 
some of his toes. If he is left with an even number, 
it must be two or four ; and in either case the animal 
would gain the firmest support by so disposing his 
toes as to admit of the axis of his foot passing be- 
tween an equal number of them—whcther it be one or 
two toes on each side. On the other hand, if our early 
mammal were called upon to retain an odd number 
of toes, he would gain best support by adjusting 
matters so that the axis of his foot should be coinci- 
dent with his middle toe, whether this were his only 
toe, or whether he had one on either side of it. 
This consideration shows that the classification 
into even-toed and odd-toed is not so artificial as 
it no doubt at first sight appears. Let us, then, 
consider the stages in the evolution of both these types 
of feet. 
Going back to the reptile Chelydra, it will be 
observed that the axis of the foot passes down the 
middle toe, which is therefore supported by two toes 
on either side (Fig. 78). It may also be noticed that 
the wrist or ankle bones do not interlock, either with one 
another or with the bones of the hand or foot below 
