Porritt: Melanism in Lepidoptera. 303 



than vayleyata. In other species, in captivity, and the 

 parents selected for the purpose, it has usually taken three 

 g-enerations to produce an almost completely melanic race, but 

 even then there has g'enerally been a very small percentage of 

 the pale forms. The usually accepted theory of the reason for 

 Melanism is that it is a protection against birds and other 

 enemies of the moths, that the pale forms being' so much more 

 conspicuous on the darkened (by soot, moisture, etc.) tree 

 trunks, the birds pick them off, and the dark specimens being 

 less conspicuous are more likely to escape detection, and so 

 survive in proportionately greater numbers to perpetuate still 

 darker forms. That Melanisn in many cases is apparently, and 

 very probably protective, is not denied, but we have as yet little 

 proof from what enemies such protection is needed. He did 

 not believe that birds feed to any extent on the larger moths, 

 and the nightjar which does, only takes them on the wing at 

 night when colour similarity with tree trunks would be useless. 

 The same applies to bats and other enemies, and indeed there 

 seems to be no evidence of any enemies which systematic- 

 ally search tree trunks for large species like Aniphydasis 

 hetuliD'ia, Boarmia repandaia, Pligalia pilosaria, Tephrosia 

 biundiilaria, and others. Moreover, many Melanic species, 

 such as Lareniia mnliisirigaria, the Melanism in which has 

 become very rapidly developed for no apparent reason, do not 

 affect tree trunks at all. This species hides in the daytime 

 among grass and other green herbage on the exposed hillside 

 meadows in which it flies, and where the herbage is so thick 

 that an assimilation of colour to the soil underneath would 

 seem of little use. The theory that smoke and humidity, 

 together with natural selection and heredity, in the manu- 

 facturing districts of Yorkshire and Lancashire has caused 

 Melanism, although likely in many instances, the exceptions 

 are so numerous that unless it is conceded that other and alto- 

 gether different influences were the causes in these exceptions 

 the theorv cannot stand.* 



* This paper was illustrated by a large number of specimens of Melanic 

 species from Mr. Porritt's cabinet, supplemented by specimens from the 

 cabinet of Mr. Samuel Walker, of York. In the discussion which followed 

 Prof. Poulton, F.R.S., Dr. Dixey, F.R.S., the Rev. C. F. Thornewill, M.A., 

 and Mr. Doncaster took prominent parts, Prof. Poulton pointing out that 

 it was not necessary that the melanic form of Abraxas grossuJariata should 

 increase because it happened to be exempt from ordinarj' enemies, for 

 neither birds nor mammals would touch it in any stage. 



1906 September i. 



