AND OF LIGHT. 243 



autumnaria. In these forms, the darkening is caused 

 either by the general colour being obscured, or by the 

 size and general intensity of the dark markings being 

 increased, or by both conditions.* In P. plilceas, as we 

 have seen, low temperature causes a lightening of 

 colour. When the response to temperature is indirect, 

 the effect is as often as not in one direction as in the 

 other, and there are generally more considerable 

 changes in the markings, as well as in the general 

 colouring. 



Upon the higher animals temperature probably acts 

 but seldom as a direct cause of variation. The white 

 coat which many quadrupeds develop on the approach 

 of winter in northern and arctic climates is probably in 

 great part a seasonally adaptive change, but it may also 

 be to a certain extent the immediate, though perhaps 

 only indirect, response to cold. This seems to be proved 

 by an observation of Sir J. Ross on a Hudson's Bay 

 Lemming. f This animal was protected from the low 

 temperature by keeping it in the cabin, and had in con- 

 sequence retained its summer coat through the winter. 

 On exposing it in a cage on deck, where the temperature 

 was 30 below zero, the fur on the cheeks and a patch 

 on each shoulder became perfectly white during the 

 first night. After another day's exposure " the patches 

 on each shoulder had extended considerably, and the 

 posterior part of the body and the flanks had turned a 

 dirty white. . . At the end of a week it was entirely 

 white, with the exception of a dark band across the 

 shoulders, prolonged posteriorly down the middle of 



* Vide Merrifield. Trans. Ent. Soc. 1892, p. 33. 

 f Appendix to Second Voyage. Nat. Hist., p. xiv., r !835. Quoted 

 from Poulton's " Colours of Animals," ed. i. p. 94. 



