100 EDWARD A. WILSON. 
saving method of progression, beautiful enough to watch, but useful only in a land 
where time-saving methods are of little value. 
If, then, invisibility is of no use to this bird either for protection or for procuring 
food, one is bound to go farther into the case and ask what other reasons can be given 
for this predominance of the white and lighter phases within the icy regions. Of the 
fact I am convinced, and I can see no explanation that can meet it on biological 
grounds. I believe, however, that it is a case which strongly upholds the physiological 
theories more than once advanced by Captain Barrett Hamilton and Mr. Bonhote 
recently, and by others long before, to account for the whiteness of Arctic and Alpine 
types.* These theories suggest that pigmentation is present most abundantly where the 
physiological vigour of an animal is at a high level, and that the deposition of pigment 
peripherally is associated with an active tissue metabolism, and is fundamentally a 
reserve or so-called “‘ waste product,” which can be called upon as a supply of energy to 
the body when occasion needs and under certain conditions. Also that economy in 
tissue metabolism tends to diminish the formation of pigment, and therefore that one 
may expect to find a diminution of pigment in any animal that is living at a 
disadvantage in respect to its surroundings. Under such disadvantages we may suppose 
that some of the Arctic and Alpine animals are living, eg., Ossifraga and Pagodroma, 
and to make up for this they have necessarily to economise their physiological forces, 
or, in other words, as far as possible to check the metabolism of their tissues. This leads, 
amongst other things, to the accumulation of fat and the reduction of pigmentation, and 
it isa notable thing, already widely recognised, that the accumulation of fat to an 
excessive degree and the absence of pigment in hair and feather, is frequently 
associated not only with Arctic and Alpine climatic conditions, but also with seasonal 
and age changes, all of which may in a sense be classed together as conditions having 
a depressing effect upon the metabolism of the various tissues, all, therefore, tending 
to check the production of so-called waste products, including pigment, all tending also 
to pallor and whiteness in the various tissues, including the hair, feathers, fat and 
skin of such animals as are exposed to them. That pigment granules can be removed 
from hair and feathers by the agency of phagocytes seems to be an established 
fact, and it accounts probably for a number of transitions from darker to paler 
tints, including those of old age and winter whitening, though the converse, by 
which white hairs or feathers convert to dark without a moult, is not established. 
And although, in speaking loosely of the “ bleaching effect” of the short Antarctic 
summer, which far exceeds that of the preceding winter, upon the fur of 
Lobodon and Leptonychotes, and the feathers of Megalestris, Aptenodytes and Pygoscelis, 
the idea conveyed is probably that of some chemical change in the pigment granules, 
it is very possible that there may have been in the course of the summer months a 
definite withdrawal of these pigment granules preparatory to the growth of the new 
* Ina paper on Winter Whitening of Animals in the Proc. Roy. Irish Acad., Vol. XXIV., Sect. B., Part 4, 
by Captain Barrett Hamilton, and in “ Knowledge,” Dec., 1905, p. 293, in a paper on Colouration in Mammals 
and Birds, by Mr. Bonhote. 
