1 04 E VOL UTION AND DISEASE. 



structures are developed, demonstrates conclusively 

 enough that feathers, hair, and teeth are specialized 

 papillae of the skin. It is not my intention to discuss 

 this question, especially as the researches of anatomists 

 have long established the truth of this generalization. 

 As a rule feathers and teeth are formed from simple 

 papillae, but occasionally a papilla will dichotomize ; 

 should this occur, the result is a bifurcated feather or, 

 in the case of the dental papilla, two teeth will appear 

 fused together. 



Most of us are acquainted only with single feathers, 

 such as are used for making pens. If, instead of the 

 large wing feathers, we select one from those covering 

 the body contour feathers, as they are called we 

 find in very many birds each quill bearing two vexilla ; 

 the second is called the aftersJiaft or hyporachis, the 

 part of the feather by which they are attached to 

 each other is the calamus or quill. In some birds, 

 such as the emu, the feather and aftershaft equal each 

 other. The two forms of aftershaft are represented 

 in fig. 52. A is from the Himalayan Monaul, and B 

 from the Emu (Dromons novce-hollandice). These forms 

 of feathers arise from dichotomy of the feather-papilla. 

 It is difficult, without specially investigating the matter, 

 to be sure whether the emu's equal-sized feathers are 

 due to equal dichotomy of the papilla, and that of the 

 monaul to unequal dichotomy, or if, in the last case, 

 the feather grows at a greater rate than the aftershaft 

 and stunts it. 



Hairs grow from cutaneous papillae in the same way 

 as feathers ; occasionally in hairy men hairs are fur- 



