VARIETIES IN EPIDERMIS 153 



upon its corrugated areas, and a response to more delicate tactile 

 experience has been thus produced by its intermittent performance 

 of ordinary progression. 



Azara's opossum presents about as large a part of the surface 

 covered with nodules as with papillary ridges, the latter highly- 

 developed for an animal so low, zoologically -speaking, but one in 

 which delicate discrimination is much practised. 



The kinkajou, another arboreal animal which walks about on 

 trees more than it uses its feet for prehension, trusting much to 

 its prehensile tail, shows its corrugated epidermis and papillary 

 ridges developed in about equal proportions. 



These five mammals thus show that the stimuli of pressure 

 and friction and the response to them are being complicated by 

 the addition of the more delicate tactile organs known as papillary 

 ridges, and these, perhaps, in a secondary way are becoming useful 

 in preventing friction. But I must not omit to point out that, 

 qua prevention of slipping, the few sparse papillary ridges of the 

 hedgehog, squirrel, kinkajou and flying-phalanger, especially those 

 on the extreme tips of the digits, could have no effect in this 

 prevention and no survival value. It is otherwise when they are 

 developed in large areas as in the succeeding groups. 



Primates. 



All the thirty species of Primates possessed papillary ridges 

 to such an extent that only small areas of the palmar and plantar 

 skin of the lemurs showed any other than these remarkable 

 characters. It is so much a property of the Primate hand and foot 

 to possess these that it might be almost made a matter of ordinal 

 rank belonging to the Primates, were it not that a few stray lower 

 mammals also possess it. 



The black-headed lemur is the lowest Primate examined and 

 it is characterised by highly developed patterns of ridges on the 

 palm and sole, and these are interspersed with nodules on the 

 regions less exposed to pressure. The complexity of the patterns 

 of another, the ring-tailed lemur, is greater still. Now these nodules 

 are distinguished from the rough undifferentiated nodules of lower 

 forms, such as the Canadian tree-porcupine, and from the scales 

 in others. When examined with a lens the separate nodules show 

 small groups of papillary ridges two, three or four on each nodule, 

 arranged in a direction parallel to those of neighbouring nodules. 

 They are in fact papillary ridges in embryo, and shortly above this 

 lemur-stage in the ascent of animal life they are merged into papillary 

 ridges in patterns. All this is well told at length by Dr. and Mrs. 



