Heredity aiid the Origin of Va7'iatio7is. 165 



assurance,* in considering a different case, " it is the white 

 of the ptarmigan (modified by climatic influence) that has 

 sent the bird to the snowy wastes and bare mountain-tops, 

 and rigorously keeps it there; not the bird that has 

 assumed, by a long process of natural selection, a white 

 dress to conceal itself in such localities." Professor 

 Eimert contends that in the Nile valley the perfectly 

 gradual transition in the colour of the inhabitants from 

 brownish-yellow to black in passing from the Delta to the 

 Soudan is particularly conclusive for the direct influence 

 of climate, for the reason that various races of originally 

 various colours dwell there. 



Mr. A. E. Wallace saysf of the island of Celebes 

 "that it gives to a large number of species and varieties 

 (of Papilionidae) which inhabit it, (1) an increase of size, and 

 (2) a peculiar modification in the form of the wings, which 

 stamp upon the most dissimilar insects a mark distinctive 

 of their common birthplace." But this similarity may 

 largely, or at least in part, be due to mimicry. Most 

 interesting and valuable are the results of Mr. E. B. 

 Poulton's experiments on caterpillars and chrysalids.§ They 

 show that there is a definite colour-relation between the 

 caterpillar {e,g. the eyed hawk-moth, Smerinthus ocellatus) 

 and its food-plant, adjustable within the limits of a single 

 life ; that the predominant colour of the food-plant is itself 

 the stimulus which calls up a corresponding larval colour ; 

 that there is also a direct colour-relation between the 

 chrysalids of the small tortoiseshell butterfly {Vanessa 

 urtica) and the surrounding objects, the pupae being dark 

 grey, light grey, or golden, according to the nature and 



* Op. dt, p. 47. I venture to say, " with some assurance," because Charles 

 Darwin, who had also considered this matter, writes, "Who will pretend 

 to decide how far the thick fur of Arctic animals, or their white colour, is 

 due to the direct action of a severe climate, and how far to the preservation of 

 the best-protected individuals during a long succession of generations ? " 

 (^' Animals and Plants under Domestication," p. 415). 



t • ' Organic Evolution," English translation, p. 88. 



X " Contributions to Natural Selection," p. 197. 



§ Since this was written, Mr. Poulton has described his results in an 

 interesting volume on " The Colours of Animals " (^.v.). 



