230 DARWINISM 



So many cases are adduced from so many different islands, both 

 in the eastern and western hemisphere, that it is impossible 

 to doubt the existence of some common cause ; and it seems 

 probable to me now, after a fuller consideration of the whole 

 subjectof colour, that here too we have one of the almost innumer- 

 able results of the principle of protective coloration. White is, 

 as a rule, an uncommon colour in animals, but probably only 

 because it is so conspicuous. Whenever it becomes pro- 

 tective, as in the case of arctic animals and aquatic birds, it 

 appears freely enough ; while we know that white varieties 

 of many species occur occasionally in the wild state, and 

 that, under domestication, white or parti-coloured breeds are 

 freely produced. Now in all the islands in which exception- 

 ally white-marked birds and butterflies have been observed, 

 we find two features which would tend to render the con- 

 spicuous white markings less injurious — a luxuriant tropical 

 vegetation, and a decided scarcity of rapacious mammals and 

 birds. White colours, therefore, would not be eliminated 

 by natural selection ; but variations in this direction would 

 bear their part in producing the recognition marks which 

 are everywhere essential, and which, in these islands, need 

 not be so small or so inconspicuous as elsewhere. 



Concluding Remarks. 



On a review of the whole subject, then, we must conclude 

 that there is no evidence of the individual or prevalent colours 

 of organisms being directly determined by the amount of light, 

 or heat, or moisture, to which they are exposed ; while, on the 

 other hand, the two great principles of the need of concealment 

 from enemies or from their prey, and of recognition by their 

 own kind, are so wide-reaching in their application that they 

 appear at first sight to cover almost the whole ground of 

 animal coloration. But, although they are indeed wonderfully 

 general and have as yet been very imperfectly studied, we are 

 acquainted with other modes of coloration which have a 

 different origin. These chiefly appertain to the very singular class 

 of warning colours, from which arise the yet more extraordinary 

 phenomena of mimicry ; and they open up so curious a field 

 of inquiry and present so many interesting problems, that a 

 chapter must be devoted to them. Yet another chapter will 



