CONDITIONS OF THE CHANGE OF COLOR. 123 



the change. But it is to be remembered in this connection 

 that the difference in temperature is necessarily coordinated 

 more or less perfectly with the progress of the seasons, so that 

 it becomes in effect merely a varying element in the periodical 

 phenomena. The question x^ractically narrows to this : Is the 

 change coincident with renewal of the coat, or is it independ- 

 ent of this, or may it occur in both ways f Specimens before 

 mej^^ove the last statement. Some among them, notably those 

 taken in spring, show the long woolly white coat of winter in 

 most places, and in others present patches — generally a streak 

 along the back — of shorter, coarser, thinner hair, evidently of 

 the new spring coat, wholly dark brown. Other specimens, 

 notably autumnal ones, demonstrate the turning to white of ex- 

 isting hairs, these being white at the roots for a varying distance, 

 and tipped with brown. These are simple facts not open to 

 question. We may safely conclude that if the requisite tem- 

 perature be experienced at the periods of renewal of the coat, 

 the new hairs will come out of the opposite color ; if not, they 

 will appear of the same color, and afterward change ; that is, 

 the change may or may not be coincident with shedding. 

 That it ordinarily is not so coincident seems shown by the 

 greater number of specimens in which we observe white hairs 

 brown-tipped. As Mr. Bell contends, temx^erature is the im- 

 mediate controlling agent. This is amply proven in the fact 

 that the northern animals alwaj^s change ; that in those from 

 intermediate latitudes the change is incomplete, while those 

 from farther south do not change at all. 



The good purpose subserved in the animal's economy — in 

 other words, the design or final cause of this remarkable alter- 

 ation, is evident in the screening of the creature from ob- 

 servation by assimilation of its color to that of the predomi- 

 nating feature of its surroundings. It is shielded not only from 

 its enemies, but from its prey as well. Another important 

 effect of the whiteness of its coat has been noted. Mr. Bell 

 has clearly stated the case : — ^* It is too well known to require 

 more than an allusion, that although the darker colours absorb 

 heat to a greater degree than lighter ones, so that dark-coloured 

 clothing is much warmer than light coloured, when the wearer 

 is exposed to the sun's rays — the radiation of heat is also 

 never greater from dark than from light-coloured surfaces, and 

 consequently the animal heat frojn icithin is more completely 

 retained by a white than by a dark covering ; the temperature 



