282 THE VARIATION OF ANIMALS IN NATURE 



the individuals are evidently not as a rule inherited. It is 

 impossible to show that this colour-response has not been 

 established by selection, but there is also no direct evidence 

 that it has. The important point is, however, that this power 

 of response provides, at least in some cases, a method by 

 which species can assume a generally cryptic coloration while 

 maintaining non-adaptive specific differences in pattern. 



Before leaving the question of specific differences in colour 

 we will briefly mention the question of colour-polymorphism 

 entirely unconnected with mimicry. Several examples (birds 

 and mammals) have been given by Elton (1927, p. 184). 

 Further examples may be found in the following papers : 

 birds (Stresemann, 1925) ; Mollusca (Crampton, passim) ; 

 Lepidoptera (Goldschmidt, 1923, pp. 145-6) ; dragon-flies 

 (Walker, 191 2, p. 29 ; Tillyard, 191 7, p. 257). Dobrzansky 

 (1924) in his study of the colour variation of the ladybird 

 [Harmonia axyridis) shows that this beetle is extremely poly- 

 morphic, the colour ranging from yellow to black, the variants 

 tending to fall into eight main classes. Most of the variants 

 are found all over the range in different proportions, with the 

 exception that in the westernmost part there is a tendency for 

 one form to dominate all the others. 



The occurrence together of two or more very distinct 

 colour-forms of a species over a large part of its range does not 

 suggest that colour in these cases is a matter of life and death. 

 And, when allied forms have patterns very like one or other 

 phase of the polymorphic species, we may further doubt the 

 adaptive significance of colour in the non-polymorphic species. 

 Fisher (1930, pp. 166-8) has attempted to explain the co- 

 existence of polymorphic forms in another way, basing his 

 argument on Gerould's (1923) work on heredity in certain 

 Pierine butterflies of the genus Colias, in which both white 

 and yellow forms of the female are known. There is some 

 evidence that the white phase is not viable in a homozygous 

 condition, and it is possible to argue that, if there is selection 

 in favour of white wing-colour, a stable gene-ratio could be 

 formed between yellow (at a slight selective disadvantage but 

 capable of existing as a homozygote) and white (slightly 

 favoured by selection but only occurring as a heterozygote) . 

 As, however, there is no evidence of such white-favouring 

 selection, the ' explanation ' appears a good example of the 



