WINTER COLOUEING. 115 



must be of the greatest importance in considering the question 

 as to how far secular variations in the distribution of tempera- 

 ture on the surface of the globe may have acted as a selective 

 influence on the animals peopling it. For instance, the selec- 

 tion from species now living, which might possibly at some future 

 time recur as the result of a considerable reduction in the tem- 

 perature of our latitudes and the introduction of new species 

 from high latitudes — the fauna of the ice period — might be 

 accompanied by modifications in the structure of the individuals 

 of such species as survived the change. 



Now, so far as I know, no single investigation proves 

 that the coarser main structure of the different organs of 

 animals can be modified by changes of temperature. On the 

 otlier hand, many modifications in the fur and colouring of 

 mammals, and in the colouring of birds and insects, may with 

 good reason be referred to the direct or indirect influence of a 

 reduced temperature; as regards insects, indeed, this efftct has 

 been positively proved by experiment. (Another effect of a, 

 falUng temperature, namely, its influence on the production of 

 eggs, will be treated of later.) 



It has been recently asserted, on the contrary, that the 

 change of colour in winter, which occurs regularly in many 

 mammals and birds, is a result not of cold, but of selection. I 

 must confess that I do not understand how svich a conclusion is 

 arrived at. It is self-evident that selection per se cannot 

 possibly modify colour, i.e. the pigment itself, in the smallest 

 degree ; the causes which result in a brown fur becoming white, 

 more or less quickly, must undoubtedly be of a diflferent cha- 

 racter. It is well known that the whitening of the hair in man 

 is usually a sign of advancing years, that sometimes it occurs 

 in early life, and that this peculiarity is often hereditary, and 

 that occasionally violent and sudden agitation of mind will 

 cause it within a few hours ; in all these cases the efficient 

 causes are perfectly dissimilar. But of the nature of these 

 causes we are wholly ignorant ; and we may, though with 

 caution and reserve, express the view that the fall of tempera- 

 ture at the beginning of autumn may somehow produce an 

 effect, direct or indirect, on the pigments situated in the skin. 



