THE COLORATION OF ANIMALS 65 



doubted whether natural selection can explain this double coloration, 

 but I do not know where the difficulty lies, and there is certainly no 

 other principle whose aid we can evoke. The mountain hare must 

 have had some sort of colour before it attained to seasonal dimor- 

 phism. Let us assume that it was brown, that the climate became 

 colder and the winter longer, then those hares would have most 

 chance of surviving which became lighter in winter, and so a white 

 race was formed. Poulton has shown that the whiteness is due to 

 the fact that the dark hairs of the summer coat grow white as they 

 lengthen at the beginning of winter, and the abundance of new hairs 

 which complete the winter coat are from the first white throughout. 

 If the white hairs were to persist throughout the summer it would 

 be very disadvantageous to their wearer ; so a double selection must 

 take place, in summer the individuals which remain white, in 

 winter those which remain brown, being most frequently eliminated, 

 so that only those would be left which were brown in summer and 

 white in winter. This double selection would be favoured by the fact 

 that there would be, in any case, a change of fur at the beginning of 

 summer ; the winter hairs fall out and the fur becomes thinner. The 

 process does not differ essentially from that which takes place in any 

 species when two or more parts or characters, which are not directly 

 connected, have to be changed, such as, for instance, colour and 

 fertility. The struggle for existence will in this case be favourable, 

 on the one hand, to the advantageously coloured, and on the other to 

 the most fertile, and though the two characters may at first only 

 occur separately, they will soon be united by free crossing, until 

 ultimately only those individuals will occur which are at once the 

 most favourably coloured and the most fertile. So in this case there 

 remain only those which are brown in summer and white in winter. 



We must ascribe to the influence of the processes of selection the 

 exact regulation of the duration of the winter and summer dress, 

 which has been carefully studied in the case of the variable hare. 

 In the high Alps it remains white for six or seven months, in the 

 south of Norway for eight months, in Northern Norway for nine 

 months, and in Northern Greenland it never loses its white coat at all, 

 as there the snow, even in summer, melts only in some places and for 

 a short time. But apart from concealment there is certainly another 

 adaptation involved here — namely, the growth of the hair as a 

 protection against the cold. From an old experiment made in 1835 

 by Captain J. Ross, and recently brought to light again by Poulton, 

 we learn that a captive lemming kept in a room in winter did not 

 change colour until it was exposed to the cold. The constitution of 



