22 EDWARD A. WILSON. 
result of the necessity for extra strength and efficiency in those parts to avoid natural 
enemies over and above their uses in obtaining food, while the bill lacks this stimulus 
to rapid growth. 
The individual variation to be found in a series of Emperor Penguins 
amounts to very little indeed. When the variation in size and weight has been 
mentioned, as illustrated in the series of thirty-three birds given above, there is 
no special point to which attention can be drawn except in the curve of the bill. Here, 
however, there is some variation having no apparent reference to sex or age. In some 
individuals the curve is decidedly more marked than in others, as may be seen, for 
example, in the two birds of a similar age in Figs. 5 and 6 of Birds, Plate III. This is 
not merely characteristic of immaturity, for the curved type shown in Fig. 5 is found 
even more strongly marked in some of the adult specimens in the collection. 
The colour of the side plate of the lower mandible is also variable, from yellow 
through orange and red to lilac; but it seems to depend to a certain extent on the 
condition of the peripheral circulation, turning to a livid lilac when the circulation 
is depressed by cold. The depth of the lemon-yellow colour of the breast and 
abdominal feathers varies considerably according to the extent to which the summer 
sun has faded it. When freshly moulted the whole of the lower parts must be de- 
scribed as pale lemon yellow, not white, as in the catalogue of the British Museum. 
In describing the immature bird also there is a point worth noting in the colour 
of the crown, which is of pale bluish grey, marked off as a distinct patch from the 
darker grey which surrounds it. This bluish patch is noticeable during the first 
seventeen months, but the head then becomes jet black all over. The point is the 
more interesting because it is exactly reproduced in the first prenuptial plumage of the 
immature King Penguin. As a grey coronal patch it is lost in both King and Emperor 
when the immature plumage is discarded, but in the King the tendency to differentiate 
in colour in this part of the head is again brought out by the deposition of minute 
quantities of golden yellow pigment in the coronal and particularly the superciliary 
feathers, which with the black gives the crown a greenish sheen. This is also to 
be seen upon the chin and throat. That this tendency to deposit golden yellow 
pigment should occur in the position in which the golden superciliary crests occur 
in Catarrhactes and Megadyptes is most significant, and an examination of a specimen 
of the King Penguin in the collection of the British Museum, now figured for the 
first time in Fig. 4 of Birds, Plate VIII., will show how far this tendency may be 
carried in a particularly vigorous individual. 
Attention is particularly drawn to the distribution of the golden yellow pigment in 
this specimen, because it has a definite bearing on the genesis of the superciliary crests 
in those genera which normally possess them. To this end I have gone into some 
detail in describing the head and neck of this particular specimen in the chapter devoted 
to Aptenodytes patagonica (pp. 35, 36), and there the question of its relationship to 
Aptenodytes forstert is dealt with more fully. 
