XXIV.] NATURAL SELECTION. 323 



the same kind (to recur to a former illustration) as that of 

 obtaining a poem by shaking letters at random out of 

 a box. 



I conclude from these facts and reasonings, that the facts Summary, 

 of organization may be accounted for in part by the direct 

 action of external inorganic forces on the organism ; in 

 part by the action of the organism itself, producing self- 

 adaptation ; and in part by natural selection among spon- 

 taneous variations : but that in addition to, and in co- 

 operation with, all these, there must be a principle of 

 Organizing Intelligence. I shall defer the further con- 

 sideration of the bearings of this conclusion to a future 

 chapter, and shall end this with some observations on that 

 very remarkable class of adaptations known as Imitative 

 Eesemblances. 



Many birds, insects, and other animals have colours re- Imitative 

 sembling those of the objects among which they live. ^° °^"^g 

 The purpose of this, beyond doubt, is protection against 

 enemies : but what is the cause ? N"atural selection is the 

 most obvious cause to assign, and in many cases it is pro- 

 bably quite adequate : — those individuals which were 

 coloured like the objects among which they lived, escaped 

 their enemies oftener than the ones not so favoured, and 

 consequently survived and bequeathed their colouring. 

 This view is supported by the fact that imitative colouring 

 is worn by no birds except those which inhabit exposed in birds, 

 places, where they would be in danger from birds of prey.'- 

 But this cannot be the purpose of the white colour of the 

 Polar bear, which has no enemies, at least on land ; its in the 

 white clothing must have been given to it for warmth, as ^°^^^ 

 white substances are those wliich radiate heat the slowest. 

 Natural selection, however, will apply equally well to this 

 case ; and if all cases of adaptive colouring were single, I 

 should think this explanation would account for them all. 

 But they are not all single ; there is the remarkable double , . ,, 

 fact of some animals, of which the ermine is the best-known ermine. 



1 Tliis is stated by tlie Duke of Argyll in " The Reign of Law." 



y2 



