454 Opposable Power of the Thumb 



loped, would never have themselves given origin to the idea that 

 they differed from the common Simiae in the opposable power 

 of the anterior thumbs. After having become once possessed of 

 this fact, however, from my observations on the genera Mycetes 

 and Pithecia, I paid very close attention to the actions of the 

 various species of Cebi which have been from time to time 

 exhibited in the Zoological, and Surrey Zoological, Gardens, 

 yet without having ever remarked them, even in a single in- 

 stance, to oppose the thumbs to the other fingers in the act of 

 prehension. It was very evident, indeed, notwithstanding the 

 fallacious appearance occasioned by the backward position of 

 the organ, that they had not the power of doing so: and, in fact, 

 their principal power of prehension seems to be altogether 

 independent of the thumb; for, generally speaking, it was not 

 brought into action at all, at least not simultaneously with the 

 other fingers, but hung loosely on one side, as I have seen it 

 do, in like circumstances, in the opossums, phalangers, and 

 other arboreal mammals. When actually brought into play, 

 however, the thumb of the Cebi invariably acts in the same 

 direction as the other fingers; and, consequently, the exception 

 which these animals might be supposed, on a casual exami- 

 nation, to offer to the general law of organisation presented 

 by the other Simiadse of the New World, is altogether illusory, 

 and vanishes when put to the test of more accurate investi- 

 gation. 



1 have not enjoyed the same opportunities of extensive 

 observation among the Callitrices : indeed, I have seen only 

 two species of this genus alive ; but their actions, as well as 

 the form and position of their anterior thumbs, were in all 

 respects similar to those of the Cebi ; nor have I any reason 

 to believe that other American Simiadae differ in this respect 

 from those which I have seen, and of which I have here 

 described the actions. 



It has been already observed that the genera Mycetes, La- 

 gothrix, A6tus, Pithecia, and Hapale have the anterior thumb 

 placed absolutely upon the same line with the other fingers, 

 and acting habitually in the same direction ; and I have now 

 shown that the backward position of this organ in the Cebi 

 and Callitrices is the only equivocal circumstance which ap- 

 pears to differentiate them from the other Simiadas ; but that, 

 when observed more closely and in action, the deception 

 vanishes, and its functions are found to differ in no respect 

 from those of the allied genera. Here, then, we have obtained 

 a new and far more important character, by which to distin- 

 guish the Simiaa of the Old and New Worlds, than those which 

 have been hitherto so much insisted upon by zoologists, the 



