BILL AND HAIRS OF ORNITHORHYNCHUS PARADOXUS. 189 
We are thus led to believe that both feathers and hairs were, 
so far as their essential structure is concerned, existent in the 
Reptilian ancestors of birds and mammals. 
Now in woodcut fig. 2 we have the representation of a scale 
which contains everything essential to the structure of both 
hair and feather—equally capable of attaining, and attaining 
by no very important changes, the simplicity of the one and 
the complexity of the other. 
On this hypothesis the hair represents the axial, its inner 
root-sheath the appendicular part of a feather; and thus an in- 
telligible morphological significance is given to the mysterious 
inner root-sheath—a true part of the hair itself, and with it 
arising from the bulb—but which, owing to the mode of 
development, is buried deeply beneath the surface. 
It is, indeed, possible that the existence of such a thick 
under-coat may have conferred upon the ancestors of homo- 
thermic mammals and birds the power of becoming themselves 
homothermic. ‘ 
In birds the non-conducting coat is still supplied by the 
appendicular parts of feathers, while in mammals the homolo- 
gous structure is not available, being invaginated into the fol- 
licle as the inner root-sheath. Under these circumstances a 
non-conducting coat has been supplied by a fresh formation of 
fine hairs. The transition was probably very gradual, and the 
latter are to be regarded as much later products than the large 
hairs. It is in harmony with these supposed changes that the 
small hairs of Ornithorhynchus depart in so many respects 
from the ancestral characters of the large hairs, and that they 
should be entirely wanting from those parts where the large 
hairs are, except as regards the medulla, the most scale-like, and 
possess the thickest inner root-sheath, and where the necessity 
for warmth is less imperative. 
The invaginated inner root-sheath has, I believe, an important 
function in retaining the hair in its follicle. The hair itself, 
growing from the soft cells of the bulb, cannot be fixed very 
firmly, but the inner root-sheath pressed tightly between the 
hair and the outer sheath, and with its innermost cells imbri- 
