8 CHARLES MIDLO AND HAROLD CUMMINS 



of their sweat glands. . . . With the lowering of the pads due 

 to adaptation of the chiridium to prehension, the area of fric- 

 tion skin increased in proportion to the enlargement of the 

 contact surface, until, in the majority of Primates, the entire 

 chiridial surface has become covered by ridges, having sepa- 

 rate scale units along the margin of the chiridium only." 



Kidd, while not studying so exhaustively the ultimate detail 

 of ridge structure, supplies descriptions of the extent of ridge 

 formation which agree closely with those of Whipple. His 

 view of the factors promoting the evolution of ridges is, how- 

 ever, a different one. He ('20) emphasizes that 



"the disappearance of the rough, plain, nodular or corrugated 

 epidermis [embracing structures antecedent to ridges] in 

 mammals is coincident with increasing activity and intelligence 

 in forms who employ or acquire a more delicate sense of touch 

 in their hands and feet. The cruder response of structure 

 to stimuli of friction and pressure, evident in the lower forms, 

 is abandoned in the higher, as tactile delicacy in prehension 

 comes more into play. ' ' 



Dankmeijer's observations on the marsupials are important 

 in the present connection. Different forms within the group 

 exhibit great diversity in the presence and extent of ridge 

 formation. He confines attention to the posterior member, 

 with the aim of simplifying the desired correlations of struc- 

 ture and function. Dankmeijer accords special stress to the 

 finding that epidermal ridges occur on the pads of all mar- 

 supials which use this member for grasping in greater or 

 lesser degree, the entire planta being continuously ridged in 

 Phalanger maculatus. In two genera of the Phascolarctidae, 

 however, ridges are lacking even though the feet are primarily 

 used in grasping; conversely, he points out, ridges occur in 

 other mammals (e.g., some rodents, a few carnivores and in- 

 sectivores) in which the posterior member is not used as a 

 grasping organ. 



In summary of the observations of the authors cited, it may 

 be stated that the simians present the most consistent and 

 most extensive ridge development. Kidd even suggests that 

 this is a "character of ordinal rank", though he makes the 



