EXTRA DIGITS AXD DIGITAL REDUCTIONS 345 



locomotion. But if the digitigrade posture be extreme, the fate of 

 the digits will depend upon the habitat of the animal. Should the 

 region ordinarily traversed offer smooth, firm footing, an animal run- 

 ning upon the tips of its toes would use chiefly the longer third digit; 

 this digit would naturally be strengthened and increased in size, while 

 the other digits, being useless, might gradually disappear. Such 

 changes have taken place in the foot of the horse, and in this way the 

 perissodactyl, or odd-toed, type of foot has arisen. As is well known, 

 the habitat of the horse family is the dry rolling plain, and what evi- 

 dence we have goes to show -that the remote ancestors of the horse 

 ranged a similar country. The fact that the digits of the horse were 

 first reduced at their distal extremities points to the same conclusion. 

 For, as we shall see, digits which do not support the weight of the 

 body. may. if the animal frequents swampy regions, keep the foot from 

 sinking too deeply. In such cases the reduction of the digits begins 

 always at the upper or proximal end. just the reverse of conditions 

 in the foot of the horse. We may conclude then that the odd-toed 

 foot resulted from digitigrade locomotion over firm, comparatively 

 level ground. 



For rapid progression over swampy ground, the structure of the 



foot must conform to two requirements. It must be prevented from 



sinking too deeply and must be easily withdrawn. Any one who has 



attempted to walk across a mud flat can appreciate the importance of 



these two factors. It is in adaptation to these requirements that the 



artiodactyl. or even-toed, type of foot has evidently been developed. 



If an ungulate was of semi-aquatic habits, or attached to swampy 



places, all four digits, by spreading, would prevent the sinking of the 



foot; its withdrawal would be facilitated by making the foot occupy 



as little space as possible, and this could be accomplished by shifting 



the proximal articulations of the outer digits inward and posterior to 



the middle digits. The toes would then be arranged in pairs, the outer 



pair lying somewhat behind the other. Xow in a semi-aquatic animal 



each pair of digits would be subjected to the same usage in walking 



or running through boggy ground; the digits of each pair would tend 



toward the same structure on this account. But as the middle digits 



would support the greater part of the strain brought to bear upon the 



foot, this pair of digits would naturally become larger and stronger 



than the other. Should our hypothetical ungulate change its habitat 



from the swamp to the firmer footing of the plain or upland, the outer 



pair of digits would not reach the ground, and unless they proved of 



use to the animal in some other way we should expect them to speedily 



disappear. 



This theory as to the origin of the artiodactyl foot is supported when 

 we examine into the habits of the even-toed ungulates. The more 

 primitive forms are attached to the water. The amphibious hippo- 



