VAKIABLE RESEMBLANCE IN VERTEBRATA, ETC. 93 



the surroundings. The simplest view to take of the 

 matter would be to suppose that natural selection has 

 favoured an extra growth of hair of a white colour for 

 the winter season, so that if an animal were trans- 

 ported to the equator, a similar change would take 

 place at a corresponding time. If the change was 

 thus merely contemporaneous and without any actual 

 physiological relation to the surroundings, it would 

 require discussion in the previous chapters, for it 

 would be precisely parallel to the darkening of the 

 larva of the Privet Hawk Moth, which takes place 

 whether it will descend upon brown earth with which 

 it will harmonise, or green turf against which it will 

 be conspicuous (see pp. 42-43) . It is possible that the 

 change of certain purely Arctic animals is of this 

 kind ; but it must be remembered that many such 

 animals range southward into districts where the 

 white coat would be conspicuous in winter, so that 

 the higher power of Variable Protective Eesemblance 

 would be very beneficial. 



The question is, however, one of evidence, and I 

 shall show that in certain species the change in 

 colour is physiologically associated with the conditions, 

 like the change in the colour of a fish which depends 

 on the reflected light entering its eye. A discussion 

 of the probable nature of the physiological association 

 is better deferred until after considering the evidence. 



