H. Onslow 297 



pupate successfully than type. That this was so, seemed to be indicated 

 by the fact that several of the type insects were too weak to emerge 

 and had to be assisted from their cocoons, whereas most of the melanic 

 insects emerged normally. Whether this will also explain the excess of 

 melanics obtained by Hamling and Harris is doubtful, especially as there 

 is no record as to whether these insects belonged to the first or second 

 broods. It does not seem improbable, however, that in their experiments 

 there was a mortality which favoured the stronger melanic insects at 

 the expense of the type. It should be mentioned that Southey^ is not 

 of this opinion, as in his experience the melanic larvae were the weaker. 

 If this explanation is correct it emphasises an important point which 

 has been mentioned before", namely the fact that many melanic forms 

 appear more robust and hardy than the corresponding type forms. 

 It is obvious that if a constitutional hardiness accompanies melanism, 

 selection would very soon cause these forms to replace type insects in 

 localities where the mutation has occurred. Other factors might assist 

 in bringing this about, but they would probably be much less important. 

 From this view less stress need be laid on the occurrence of melanic 

 varieties in manufacturing districts, and the numerous black forms that 

 have arisen in the rural districts of the south and elsewhere do not 

 appear anomalous. 



1 Southey, loc. cit. (see p. 294). 



- Onslow, H., Journal of Genetics, Vol. ix. No. 4, 1920. 



