CH, X.] NATURAL SELECTION. 21 



other six mouths above the horizon ; and though its slanting 

 rays do not cause excessive heat in the summer, the prolonged 

 glare of light, intensiiied by reflection from the snow and ice, 

 is described as peculiarly intolerable. The summer ought to 

 tan the polar bears as much as the winter can bleach them. 

 And to this it may be added that the Eskimos and Green- 

 landers, living under the polar circle, are not bleached. 

 Several other facts, alike incompatible with the direct action 

 of physical agencies, are mentioned by Mr. Wallace. While 

 wild rabbits, for instance, are always tinted grey or brown, 

 the same rabbits, when domesticated, give birth to white and 

 black varieties, though there has been no change either in 

 climate or in food. The case is the same with domestic 

 pigeons. But even supposing that the most general features 

 of animal colouring could be explained on this hypothesis — 

 which they cannot be — there would, still remain the more 

 remarkable cases of tree-frogs, which resemble bark, and of 

 the so-called leaf-butterflies, which when at rest are indistin- 

 guishable from leaves ; and the existence of such cases is a 

 stumbling-block in the way of all theories save the theory of 

 natural selection. 



For according to the theory of natural selection each species 

 of animals will be characterized by that shade of colour which 

 is most advantageous to the species in the struggle for exist- 

 ence. Now, as Mr. Wallace observes, " concealment is useful 

 to many animals, and absolutely essential to some. Those 

 whicn have numerous enemies from which they cannot escape 

 by rapidity of motion, find safety in concealment. Those 

 which prey upon others must also be so constituted as not to 

 alarm them by their presence or their approach, or they would 

 soon die of hunger." In striking harmony with this general 

 principle, we find that the great majority of a/iimals are so 

 coloured as best to escape notice, and that animals which are 

 not protectively coloured are animals whose habits of life are 

 Buch as to enable them to dispense with secrecy. The polar 



