214 THE VARIATION OF ANIMALS IN NATURE 



Bateson's contention (19 13, p. 138) that between doubledayaria 

 and the typical form there are few if any intermediates. 

 Three explanations of this history are available. 



(a) Protective value of the dark colour in industrial districts. 



It has been suggested that the dark colour affords a pro- 

 tective resemblance (against birds) to smoke-darkened foliage, 

 etc., in the industrial districts in which it undoubtedly arose. 

 This has been answered by Bateson, who, reasonably enough, 

 points out (i) that doubledayaria is conspicuous anywhere 

 except on actually black materials, and (ii) that it occurs in 

 country districts between the towns. Bateson's criticisms 

 overlook the possibility that, if even 1 per cent, of the double- 

 dayaria were protected when on very sooty or dirty back- 

 grounds, it would give them an advantage. Furthermore, 

 Mr. A. W. McKenny Hughes informs us that Bateson very 

 much minimises the concealing effect of the dusky colour, 

 which Mr. Hughes asserts is marked. It should be noted that 

 Kane (supra, p. 202) claims that dark forms of moths are 

 protectively coloured on certain rocks on the coast of 

 W. Ireland, and have multiplied accordingly. 



(b) Greater viability, etc., of the melanics. 



Bowater (19 14, pp. 300, 303, 308) states that in the course 

 of breeding experiments on Spilosoma lubricipeda and other 

 forms the melanics are larger and stronger than the type and 

 are double-brooded. This was not actually observed by him 

 in betularia, and, in view of the capricious incidence of physio- 

 logical variation, we would hesitate to assert that it is likely to 

 be also found in that species. 1 It nevertheless remains a 

 possible explanation. 



(c) Harrison's theory. 



As far as the actual origin of the melanic character is 

 concerned, Harrison and Garrett (1926) and Harrison (1928) 

 endeavoured to show that it was due to the salts contained in 

 the soot-covered food in industrial areas. They did not 

 commit themselves to theorising how the mutants spread 

 beyond the industrialised area. 



1 Harrison (1928) stated that the artificially produced melanics of other moths 

 are more delicate than the typical form. 



