THE GIANT PETREL. 101 



hair or feathers which are to replace those that contaiued them, and this view is 

 to a slight extent upheld by a microscopic examination of fresh-grown and of so-called 

 " bleached " hairs of Lohoclm, for there appears to be not so much a change in the 

 colour and character of the pigment granules, as a simple diminution in their number, 

 though to make this point certain it would, of course, be necessary to repeat on the 

 animals in question the observations made by Professor Metchnikoif on men and dogs 

 (Proc. Roy. Soc, London, Vol. LXIX., p. 156, 1901). 



But in 0. gigantm we have to deal, not with a seasonal change of any kind, but 

 with an increasing tendency in a pigmented species, which is becoming acclimatised to 

 the Antarctic climate, to become white and unpigmented, and I consider that there is 

 sufficient reason for discarding, the usual explanations given in such cases, seeing that 

 there is no apparent need for invisibility, and for considering whether the above physio- 

 logical reasons may not be sufficient. The bird is normally an inhabitant of the temperate 

 regions, and is apparently ranging more and more into the Southern Polar seas. In 

 doing so it becomes migratory, and each year, probably, increases the extent of its 

 winter range in point of numbers. Its tendency being towards acclimatisation for 

 severer conditions, presumably some pressure is acting from the north to check its 

 range in that direction. Every reserve of energy has to be called in to meet the extra 

 tax upon its physiological forces, and the peripheral pigmentation, even though it may 

 have had a purpose in other climates, is now recalled, while the tissue changes are 

 pari passu depressed by the climatic condition, and fat, possibly as a consequence, 

 begins to accumulate, serving a useful purpose by retaining the body heat. It is 

 just possible, moreover, that the feathers themselves, when the spaces previously filled 

 by pigment granules are occupied by air, are really a better non-conducting covering 

 for the bird than they were before. If this be so, it is an additional factor in 

 the rapid production of a white form typical of the Antarctic ice. It may be urged 

 that this white form, if becoming acclimatised under less advantageous conditions than 

 those under which the dark forms farther north exist, should show some definite 

 deterioration, just as the most southern Skua, M. maccormicki, shows a definite 

 deterioration in size from M. antarctica. Instead of this, it maintains its size, 

 and even in some small points exceeds the measurements of the darker forms, 

 suggesting that the formation of this new type or incipient species is not so much 

 the result of pressure from the north as a proof of a tendency for the more robust 

 of the race to spread in a new direction where the drawbacks may be outbalanced 

 by better food, the absence of competition or enemies, or othei' and more subtle 

 advantages at present beyond our ken. 



