BILL AND HAIRS OF ORNITHORHYNCHUS PARADOXUS. 18] 
If the cells of the bulb were to form the merest trace of a 
hair at the surface, and were then carried down in the sinking 
follicle, the hair rudiment descending in the axis would 
necessarily leave a lumen behind it—an open tube connected 
with the surface. If such were the case there would be no 
morphological distinction between hair and feather, for the 
follicular downgrowth would still be secondary in the hair as 
it is in the feather, although preceding the development of 
everything except the hair tip. And such a hair, developed, 
except for its tip, from an epithelial upgrowth rising from the 
bottom of an open tube connected with the surface, would still 
be, from a morphological standpoint, at the surface, and would 
be entirely and strictly homologous with a structure developed 
from an epithelial upgrowth rising from the surface. 
This will be admitted, and yet the transition from such a 
condition to that now met with in Mammalia generally is so 
easy that no important morphological distinction can be 
founded on it. It is merely that the same tendency which has 
- gradually increased the relative proportion of development at 
the bottom of the tube carries this process one small step 
further, so that the whole hair is formed beneath the 
surface. There would then no longer be such a slight up- 
growth from the bottom of the sinking tube as would ensure a 
free lumen, and sooner or later a shorter simpler method, 
leading to a solid downgrowth, would replace the open follicle, 
But all this does not imply any great morphological distinction. 
The case is, in fact, exactly parallel to the development of a 
gland—which at an earlier stage was produced from a tubular 
depression of the surface—from a solid downgrowth which sub- 
sequently becomes tubular; and the chief principle at work 
would probably be the same in both cases, viz. the tendency 
towards abbreviation and simplification of development. 
As a confirmation and a test of this argument it is important 
to inquire whether the solid cylinder above the primitive bulb 
forms any part of the hair itself, or whether it gives rise only 
to the epidermic walls of the follicular tube (outer root-sheath) 
which is afterwards formed. If the solid cylinder represents 
