158 W. A. KIDD, ESQ., M.D., B.8., M:R.C:S.; F.Z.8., ON 
The feathers being set closely together and in different 
layers, one can see how a considerable bulk of warm air is 
retained round the main organs of the body. On the other 
hand, feathers, being very light themselves, assist materially 
in lessening the specific gravity of the body; and in 
addition to this they further diminish the specific gravity by 
retaining a certain amount of warm air, serving a purpose, im 
a more delicate manner, which the “swim bladder” of a 
fish does in its element. The construction of a feather is 
elaborate and marvellously complete for the purpose. 
It cannot be better studied from the present point of view 
than in the pages of Paley. One only need refer to 
the names of the various parts of a feather. The shaft 
is divided into quill and rachis, the latter giving off numerous 
processes called barbs. These are interlocked with one 
another by the delicate contrivance of barbules held together 
by fine hooklets. These together constitute the vane of 
the feather. One small point in the quill may be noticed, 
viz., that this tough, light structure is filled with air, and a 
small opening at the distal end called the “superior umbilicus” 
is furnished so that the air inside the quill communicates 
with the outer air, thus preserving the due degree of air- 
pressure in the cavity of the quill. All this mechanism 
testifies strongly, I think, to intelligence in operation, 
whence alone structures so light, so strong, so firm to resist 
pressure, so protective against heat, cold, and moisture as: 
the commonest wing-feathers of a common bird could have 
proceeded. It may be added that the distribution of feathers 
is admirably adapted to a maximum of protection, warmth, 
lightness, and smoothness of contour, and a minimum of waste 
of tissue or size. In this connection one must mention shortly 
the remarkable oil-gland found in most aquatic birds. 
Anything more significant of design in the efficient protection 
of the feathers and body of an aquatic bird than this 
receptacle, placed in a convenient situation for the bird 
to reach it with its beak, it is hard to imagine. The small 
sac referred to lies in a sheltered, convenient position on 
either side near the tail of the bird within easy reach of its 
beak. It is well protected from pressure, and yields when 
squeezed a thick oily secretion, wherewith the bird “ preens ” 
its feathers, conferrmg upon them the necessary amount 
of “ waterproofing,” renewable as required when the bird’s. 
sensations inform it of its needs. Here is a case in which 
the protective structure is itself protected, reminding us 
