Palceontology. 183 



whether an animal has an even or an odd number 

 of toes may seem a curiously artificial distinction 

 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 — whether 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 



