104 THE COLOTJKS OF ANIMALS 



ably indicates that it is, as we should expect, at a 

 disadvantage, and that it will disappear. Mr. Wallace 

 considers that the dark colour of arboreal northern 

 animals, which is clearly for concealment, disproves 

 the theory that the white colour is of value in retain- 

 ing animal heat. But it does not follow that such 

 benefits are wholly non-existent, because they must 

 be dispensed with under the pressure of a stronger 

 necessity. Mr. Wallace's argument shows that con- 

 cealment is the paramount necessity ; but this does 

 not disprove the opinion that other advantages also 

 may be conferred by one particular mode in which 

 concealment is attained. 



The seasonal change of colour in northern birds 



The same convincing evidence as to the nature of 

 the .change, and the manner in which it is brought 

 about, has not yet been brought forward hi the case 

 of birds. Mr. A. H. Cocks, who has had a very wide 

 experience of northern animals, believes that it is at 

 least partially due to a change in the autumnal 

 feathers. He writes : ' I have some specimens of 

 Lagopus (various species) showing brown feathers 

 with white tips, and in one species, at any rate, the 

 converse.' Mr. E. Bowdler Sharpe does not however 

 think that the evidence of a winter change in existing 

 feathers is sufficient. 1 He has nevertheless proved 



1 H. Seebohm thinks it ' possible that the white winter feathers 



