COSMIC PHILOSOPHY 



of physical agencies, are mentioned by Mr. 

 Wallace. While wild rabbits, for instance, are 

 always tinted gray 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 suppos- 

 ing that the most general features of animal col- 

 ouring could be explained on this hypothesis, — 

 which they cannot be, — there would still re- 

 main the more remarkable cases of tree-frogs, 

 which resemble bark, and of the so-called leaf- 

 butterflies, which when at rest are indistinguish- 

 able from leaves ; and the existence of such 

 cases is a stumbling-block in the way of all the- 

 ories save the theory of naturd selection. 



For according to the theory of natural se- 

 lection each species of animals will be charac- 

 terized by that shade of colour which is most 

 advantageous to the species in the struggle 

 for existence. Now, as Mr. Wallace observes, 

 " concealment is useful to many animals, and 

 absolutely essential to some. Those which have 

 numerous enemies from which they cannot es- 

 cape by rapidity of motion, find safety in con- 

 cealment. 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 



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