B. -Hamilton — Winter Whitening of Mammals and Birds. 313 



a profound influence upon current Yie"ws of animal colouration. For 

 instance, it at once dawned upon me that therein lies the explanation 

 of the white "blaze" of so many domestic animals, and in particular 

 of horses. This is usually situated over the frontal or nasal hones 

 where they lie directly under the skin. Again, the fact that in man 

 baldness occurs first in corresponding regions is almost certainly but 

 another instance of the working of the same law. 



Although thus pushing my theory to lengths which have, I believe, 

 been untouched by any other view, I must be the first to point out its 

 own restrictions. I have at present, at all events (although I confess 

 I begin to see light here also), no desire to connect it with such 

 complicated colour schemes as the spots of the Leopard or the stripes 

 of the Tiger and Zebra. It is further evident that puzzles like the 

 curious arrangement of the white areas on the tails of birds, or the 

 restriction of pigment to the upper side of a flat-fish, are phenomena 

 which, although probably connected in their origin with subcutaneous 

 fat, seem to require some further factor for their full explanation. 



In bii'ds, for instance, without entering into detailed descriptions of 

 what is perfectly well known to naturalists, the light patterns on the 

 rectrices are frequently the sum of a series of complicated markings, 

 different as to each individual feather, but fitting into their place like 

 the pieces of a mosaic. Now, although the deficiency of pigment in 

 this case is, on my showing, certainly connected in a general manner 

 with the fat-tract of the region whence these feathers spring, it is 

 hard to see how all the complicated details of the pattern can be thereby 

 explained. But in view of Dr. Finsen's discoveries, it does not seem 

 too great a stretch of imagination to suppose that the exact distri- 

 bution of the pigment may be not unaffected by the varying amount 

 of light to which the different parts of the feathers are subjected, 

 or again that the pigmentary differences between the two surfaces 

 of a flat-fish may have in a like manner been due, although exactly 

 how we do not understand, to unequal stimidations of the light 

 which they receive. 



Thus, then, I have no wish to extend my arguments universally 

 to white colouration in nature, since there may undoubtedly be causes 

 other than atrophy which result in absence of pigment. It is obvious 

 also that many animals are not subject to the hair-atrophy which in 

 others follows the peripheral accumulation of fat. On the other 

 hand, it may well behove Zoologists to consider not only the external 

 advantages accruing from but the deep-seated physiological processes 

 involved in seasonal colour changes. Even those connected with sex 



