28o gidlEy: primitive mammalian foot 



come directly the modern ungulate types of feet. Still another 

 type developed from the primitive terrestrial mammalian foot, 

 or possibly directly from the reptilian stage, is that type of which 

 the short-toed amblypod and probosicidian feet are examples. 

 In these forms the toes remain more or less spreading, and all, 

 including the first when present, function in bearing the weight 

 of the body. In this type of foot the heel may become raised 

 from the ground as in the digitigrade type, but this modification 

 is always accompanied by the development of a pad so that this 

 portion of the foot still functions in carrying its share of the body- 

 weight as in the plantigrade type. It may be further assumed 

 that the various aquatic types of feet are directly derivable from 

 some such foot as that of Claenodon. 



It would thus seem that the primitive mammalian foot, of the 

 central group, must have been primarily terrestrial, and from this 

 generalized type of foot, with divergent but not primarily op- 

 posable first digit, have been developed all sorts of modifications of 

 foot structure, each adaptable to the kind of environment chosen, 

 and that divergence of the first digit is primarily an inheritance 

 from the primitive reptilian condition, and can not be considered 

 as in any way supporting the hypothesis of an arboreal ancestry 

 of the Mammalia. It is likewise quite as apparent, it seems to 

 me, that true opposability of the first digit wherever found 

 should be considered a direct specialization of the primitive 

 condition brought about by a mechanical adaptation to a pe- 

 culiar (arboreal) life habit. 



This viewpoint is certainly not weakened and seems to be 

 strengthened by the difference in type of modification observed 

 in the hind foot of marsupials as compared with that of the 

 Primates. In the former, as observed by Huxley, opposability 

 of the first digit is accompanied with enlargement of the fourth 

 digit, and reduction and syndactylism of the second and third 

 digits, while in the latter there is no approach to either of these 

 modifications. This difference seems to be fundamental, and 

 suggests that the marsupials took to tree-living habits at a some- 

 what earlier stage of development while the fourth digit was 

 yet considerably longer than the others, and for that reason 

 more directly opposed by the first digit. 



