gidlEy: primitive mammauan foot 277 



been found in any forms of older age than the middle Eocene; 

 and here it is known only in species of a single order. These facts, 

 together with the absence of opposability in tree-living rodents and 

 insectivores, as already pointed out, would in themselves suggest 

 that this condition of opposability is relatively modern in develop- 

 ment, or at least not primitive; and in further support of this 

 view it may be noted that in those orders in which opposability 

 has been developed and retained, it is always most advanced in 

 those species which are in other respects notably specialized. 

 Thus, in the Primates, opposabiHty, especially in the hind feet, 

 is found best developed in the heavier, long-limbed monkeys and 

 apes. It has reached its greatest perfection in the hand of man, 

 although doubtless the higher stages of perfection of this function 

 were accomplished after man, formerly arboreal, had finally taken 

 to a terrestrial habitat. Contrary to what one might expect, 

 if opposability were a primitive condition, in the little marmo- 

 sets of South America, which among living Primates are con- 

 sidered a rather primitive and generalized group, there is, as 

 already stated, not the slightest approach to opposability of the 

 pollex, which is long and functional and it is only moderately 

 developed in the hallux. Yet these little animals are as strictly 

 arboreal in habits as any group of the order. If opposabiHty of 

 the first digit is a primitive condition, then why is it so poorly 

 developed in this particular group while so well developed and 

 perfected in the more highly specialized members of the order? 

 Granting that all Primates began their career in an arboreal 

 habitat, it seems to me a more logical conclusion that the little 

 light-bodied marmosets have never developed opposability, 

 finding the primitive sharp claws sufficiently effective for cling- 

 ing to the bark of trees, while the heavier-bodied forms, or those 

 species which early formed the habit of swinging from limb to 

 limb as a method of progression through the trees, very quickly 

 took advantage of the primitively divergent first digit to develop 

 opposability. This function once developed to a degree where 

 the grasp became firm, the claws would no longer act as formerly, 

 and the constant pressure thus transferred to the palmar side of 

 the toe, or finger tips, would soon modify them into the "nail," so 

 characteristic of the Primates, 



