THE ADELIE PENGUIN. 55 
suggest that the tendency to vary lies in the distribution of black and white about 
the head and neck, which is exactly what one would expect, because it is the 
recognition area, so to speak, of the various species of penguin—the part which alone 
is open to view as the bird is floating on the water. The rest of the body would 
hardly suffice to differentiate a dozen species, and it would be no easy task to 
differentiate between the birds at sight if the heads were taken off at the shoulders. 
It is in the head that we have the most marked differences in the build and colour of 
the beak, wide differences in the colour of the iris; white throats in some and black in 
others, white bands across the forehead, black bands again across the throat, and a 
variety of shading and arrangement in the golden superciliary plumes and frontal 
crests, as though from time immemorial variations in the head had been selected and 
developed, and the process still goes on. 
Hence we get uniformity in those parts which are useless for specific distinction, 
but are probably of an advantageous invisibility under water; and a tendency to 
variation chiefly in those parts which carry specific recognition marks. But one may 
argue that, if the distinctive specific characteristics are to be looked for in the head, 
there is every reason why the head marks should not vary, and it is certainly hard to 
see why in any particular part there should be associated at once the necessity for 
uniformity and a special tendency to variation. 
One must allow that whereas in the parts which do not show above water the 
colouring is governed mainly by the necessity for protective invisibility, yet in the parts 
that do show above the water there is an inherent tendency to vary, from the very 
fact that the physiological processes leading to pigmentation have become concentrated 
in them ; a concentration which results from the need for recognition marks in the parts 
that do come easily into view as the penguin floats on the surface of the water. That 
the under parts should all be white seems naturally to result from the necessity for 
invisibility from below, where lurk, not only the fish that the bird must catch, but 
the seals and whales that would prey upon the bird. 
Whatever may be the reason, the fact remains that the demarcation line between 
the black and white is seen to vary about the head and neck more frequently than 
elsewhere. And the only other variation noticed in this species was in the case of 
the foot illustrated in Fig. 6 of Plate X., where the blackening of the sole was 
irregular, and also in the extent of the black tip to be seen on the under surface of 
the wing, which varies within wide limits, and which, being common to several species, 
cannot be considered of much specific value. 
There is still one other variation of the Adélie Penguin that must be mentioned. 
It is the pale or isabelline variety, of which we obtained a specimen in each of two 
successive years at the Cape Crozier rookery. Both were adults, and in both the parts 
which should have been black and bluish black were buff or brownish-buff ; the feet 
were pale flesh colour, the nails light brown and the iris also brown. 
The head of this bird is figured in Plate IX., Fig. 6, and in comparing it with 
