104 EVOLUTION 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 
papilla, 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 aftershaft 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 (Dromeus nove-hollandie). 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 papillz in the same way 
as feathers; occasionally in hairy men hairs are fur- 
e 
