gidIvEy: primitive mammauan foot 279 



maining on the ground, so that normally the weight of the body 

 is not brought fully on the ends of any of the metapodials. In 

 terrestrial mammals, even of the most primitive plantigrade 

 type, the feet are brought more under the body in walking, the 

 radius and tibia, respectively, are brought to the inside, the first 

 digit pointing constantly inward and the others more or less 

 directly forward. The result of this modification would be to 

 bring the weight of the body more exactly on the ends of the 

 median toes, especially at the finish of the stride at which time 

 the heel is raised clear of the ground. This would have a tendency 

 to stiffen the wrist and ankle joints, through a closer articulation 

 of their bony elements, to bring the divergent metapodials more 

 closely together, and to shorten the median pair of toes. This 

 change in position of the feet and consequent change of the 

 manner in which the toes are applied to the ground in walking 

 may have been the primary cause of the reduction in phalanges 

 of digits III and IV, and would quite satisfactorily account for 

 the divergence of the first digit so frequently found in the prim- 

 itive mammalian foot. It will be especially noted that the 

 first digit is shortest, and in the mammalian position of the foot 

 is so placed as to take no considerable part in the function of 

 walking, hence it has been least modified and soonest lost in 

 terrestrial forms which acquired a digitigrade gait. 



It is plainly obvious that, from a central type of primitive 

 mammalian foot similar to that just described, in which the 

 first digit is unreduced, it is but a short step to true opposability. 

 The divergent first digit could readily be converted into a grasp- 

 ing organ, and the modification to opposability doubtless was 

 soon accomplished by those forms which early adopted an ar- 

 boreal habitat. But among ground-living forms, in the other 

 direction, it was an equally short step to the strictly terrestrial 

 digitigrade type, in which the first digit was gradually atrophied, 

 as it became functionless through being raised from the ground, 

 and in many species has been lost altogether. In the process of 

 development of the digitigrade type the functional metapodials 

 are brought closely together and have become relatively length- 

 ened while the phalangeal portion of these digits becomes short- 

 ened. It is through these stages of development that have 



