B. -Hamilton — Winter Whitening of Mammals and Birds. 311 



to arrange themselves in accordance "with, one or two definite patterns. 

 The latitude is no doubt due to the fact that, although following the 

 general order described [above, the ^;awwzcw/?<s adiposus of domestic 

 animals is almost universally distributed over the body, and varies 

 only with each animal's individual idiosyncrasy of constitution. It is 

 noticeable, moreover, that the Hereford breed of cattle, in which the 

 arrangement of the fat differs from that of other breeds, it being 

 mainly distributed peripherally, is distinguished by regularity of 

 pattern, having the principal peripheral fat-tracts clearly mapped out 

 in white. 



The accumulation of fat in a fattening ox is, however, marked, 

 not by loss of pigment, but of the hair, the skin becoming bare, par- 

 ticularly on the rump and neck, as the animal ripens. This, then, is 

 only another aspect of the atrophy which may accompany deposits of 

 fat under the skiu. Here again I find more than a mere coincidence in 

 the fact that the bare buttocks of monkeys correspond to the light 

 rump-patches of many other vertebrates ; further, that the accumu- 

 lation of subcutaneous fat in marine mammals is correlated with 

 deficiency of hair, in a graduating series, from the amphibian warmly 

 furred Fur-seals to the completely aquatic hairless Cetacea and Sirenia. 



A great difficulty for some time lay in the way of my theory, 

 namely, the occasional reversal of the ordinary arrangement of 

 vertebrate colouration, whereby the ventral is usually the lighter, the 

 dorsal the darker surface. For instance, in the Skunks, Polecats, 

 and the Eider Duck, the upper surface is conspicuously lighter than 

 the under. These facts were not at all explained by Mr. Thayer's 

 hypothesis, and each case is usually argaed on its own merits, the 

 Skunk's white back being regarded as a warning of its bearer's 

 malodorous nature, the Eider Duck's as protective to the sitting-bird, 

 and so on ; they certainly proved a stumbling-block to me. I hardly 

 felt bold enough to predict that the unusual arrangement would be 

 found to correspond with a like internal disposition of the panniculus 

 adiposus ; and no other supposition, unless, as it were, some deus ex 

 machina., in the shape of an ingenious secondary explanation, seemed 

 likely to be able to pull me round the difficulty. 



A second difficulty lay in the fact that the heads of vertebrates are 

 very frequently the centre of conspicuous light marks or bars, which, 

 while not apparently related to any internal fat-tracts, are yet so 

 similar in many widely-distinct forms (such as mammals and birds) as 

 to be without doubt due to some similar cause in all. 



Most fortunately both these difficulties were simultaneously and, 



