BUTTERFLIES AND MOTHS 



to what is called environment — and it is here that the 

 butterflies and moths provide excellent illustration of 

 evolution. 



When I was a boy the common peppered moth was known 

 to produce a rare black variety. The growth of the manu- 

 facturing districts of Lancashire and Yorkshire has greatly 

 increased the volume of the smoke there, which, carried by 

 the prevalent southwest winds, has done deadly work over 

 a wide area, killing the gray lichens and leaving the tree- 

 trunks dark and sooty. Resting on bark like this the pep- 

 pered moth would be more conspicuous to the eye of a 

 bird seeking food than the black variety, and accordingly 

 for many years this black form has entirely replaced the other 

 form in these northern tracts. The others, being more easily 

 seen, have been eaten. And the peppered moth is not the 

 only species that shows change; several other bark-haunting 

 moths have also become much darker in the same strip of 

 country and during the same short period. Furthermore, 

 similar changes have been observed in the moths of other 

 smoke-producing areas in this country and on the Continent. 

 Harrison has recently shown that some of these moths have 

 become dark after their caterpillars have been fed for many 

 generations on plants contaminated with salts of manganese, 

 such as are contained in smoke. Inasmuch as the effects were 

 transmitted in Mendelian proportions we must conclude that 

 the salts acted upon the germ cells. 



A still better but less well-known example of change in 

 colour is found among the butterflies of tropical America. In 

 each district these insects and some of the day-flying moths 

 form groups that are of similar pattern and colouring but 

 that are composed of species having very different degrees of 

 relationship. Among the groups of any locality one species 

 is generally predominant in numbers and is among the most 



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