100 EDWARD A. WILSON. 



savin or method of progression, lieautiful 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 Vnological 

 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 metal)olism, 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, c.y., 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 is a 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, Ijut 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 graimles 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 " lileachiug effect " of the short Antarctic 

 summer, which far exceeds that of the preceding winter, upon the fur of 

 Lobochni and Lejitonychotes, and the feathers of Megalestris, Aptenodytes and Pygoscelis, 

 the idea conveyed is probably that f>f 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 graimles preparatory to the growth of the new 



* In a paper on Winter Whitening of Animals in the Proc. Eoy. 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. 



