276 gidley: primitive mammauan foot 



Among the Primates themselves there is a whole group of living 

 species, the South American marmosets, that, in the fore feet at 

 least, show no evidence of opposability, although they are strictly 

 arboreal in habits. These smaller, lighter-bodied animals 

 seem never to have acquired the function of grasping a limb, but 

 depend rather on their sharp, widely spread claws for support in 

 progressing among the tree tops. 



Matthew's second and third propositions, above quoted, do 

 not strictly concern opposability but refer rather to simple di- 

 vergence of the first digit. And the crux of the whole proposi- 

 tion seems to lie, after all, in the interpretation of this condi- 

 tion. 



"Primitive opposability" and "more or less opposable" are 

 terms which the advocates of arboreal ancestry have frequently 

 used, but have never clearly defined. These terms seem to ex- 

 press a condition somewhat different from the kind of opposa- 

 bility developed in the modern Primates, and as used by Matthew, 

 as I interpret it, seem to imply that simple divergence of the 

 first digit may be taken as proof of opposability or at least former 

 opposability. It becomes necessary then, to distinguish clearly 

 between "opposability" and "divergence" of the first digit, as 

 for an intelligent discussion, one should understand clearly what 

 is meant by "opposability." As applied to arboreal adaptation 

 opposability can imply but one condition, viz., a modification 

 which gives the power to grasp or hold, by opposing the first 

 digit to the others, and this is always accompanied by a special 

 and distinctive arrangement and development of the digital 

 muscles. Opposability, it is true, is usually accompanied by a 

 complete divergence of the opposing digit, ^ but divergence, in 

 all stages, is frequently observed where there is no other evi- 

 dence of opposability. Moreover, examples of divergence 

 without opposability are found most frequently in the older 

 Eocene representatives of almost all the orders of mammals 

 in which the feet are known, while true opposability has not 



* An exception to this is seen in some of the phalanges in which the first meta- 

 carpal is closely appressed to the second, but the toe is opposable and divergent 

 in its phalangeal portion. 



