244 THE EFFECT OF TEMPERATURE 



the back." No further change took place, and the ani- 

 mal died of the cold a few days later. Examination of 

 the fur showed that only the tips of the hairs had be- 

 come white, so that on cutting these off, the coat re- 

 gained its original dark colour. 



The observations of F. H. "Welch * on the American 

 Hare (Lepus Americanus) throw further light on the 

 nature of the change. Early in October the whiskers 

 and a few of the longer hairs on the back were observed 

 to become white at the tip or throughout. During No- 

 vember a new and rapid growth of stiff white hairs ap- 

 peared on the sides and back, these hairs being easily 

 distinguishable from the autumnal hairs which were 

 gradually turning more and more white, in that they 

 were invariably white throughout. We have in this 

 animal, therefore, a new white crop of hairs of gradual 

 growth, or a blastogenic variation, stimulated to de- 

 velop under stress of cold, and a rapid and direct trans- 

 mutation of parts of the dark hairs to white; i. e., a 

 somatic modification. Professor Poulton f explains this 

 latter change as an indirect influence of cold upon the 

 nervous system which presides over the nutritive and 

 chemical changes involved in the growth of the hair. 

 This probably leads to the production of large numbers 

 of gas bubbles in the hairs, and thereby induces an ap- 

 parent whiteness, in spite of the fact that the pigment 

 is still present. In that the tips of the hairs are first 

 affected, however, rather than the bases, it seems to me 

 possible that the cold acts directly on the hairs them- 

 selves, and not indirectly through the nervous system. 



*Proc. Zo5l. Soc. 1869, p. 228. 

 f " Colours of Animals," p. 100. 



