50 MOSTLY MAMMALS 



A somewhat different type of arrangement obtains in 

 the hand of the South American spider-monkeys (fig. B), in 

 which the thumb is wanting. In this group, although whorl- 

 like patterns are observable in the interdigital eminences, yet 

 they are much smaller and less distinct than in ordinary 

 monkeys ; the same being the case with the ulnar eminence. 

 The radial pattern at the inner side of the thumb is, 

 however, practically wanting, owing doubtless to the absence 

 of that digit. It will further be noticed from an examination 

 of the figure that elsewhere on the palm, not even excepting 

 the fingers, the general arrangement of the ridges is longi- 

 tudinal. Since the hands of the spider-monkeys are, as 

 already mentioned, largely used in a hook-like manner 

 during the arboreal evolutions of these active creatures, it 

 would seem at first sight that the arrangement of the ridges 

 precisely controverts what has been said above as to their 

 being parallel with the long axis of the object grasped. 

 But the palms of even these monkeys, as is indicated by 

 the numerous creases, are evidently much folded laterally; 

 and it must also be borne in mind that an equally important 

 function of the hand is the plucking and holding of spherical 

 or sub-spherical fruits. And for such a combination of 

 functions the mode of arrangement of the ridges is doubtless 

 the one most suitable. If the ridges were transverse, the 

 fruit would very probably have a tendency to slip out of 

 the hand on one side or the other ; but this is clearly 

 prevented by the longitudinal arrangement. 



The above are the chief modifications displayed by the 

 palm-prints of monkeys ; and it may be added that a very 

 similar general plan of arrangement of the papillary ridges 

 and grooves obtains on the sole of the foot of these animals, 

 subject, however, to such modification as is necessary for 

 the different function of the foot as compared with the 



