250 THE entomologist's record. 



von Linden has shown that the effects of temperature can be imitated 

 by influencing the metabolism during the pupal stage, and it is also 

 fairly certain that by artificially varying the length of pupal life, a 

 greater or less pigmentation may be produced. It has frequently 

 been assumed that melanism may be the result of changed environ- 

 ment acting on the individuals exhibiting it, and the experiments 

 referred to suggest that this may be true in some cases. But breeding 

 experiments prove that in the great majority of cases melanism is 

 exactly comparable with any other form of " spontaneous " variation 

 or dimorphism ; it is not produced by the direct action of environment 

 on the individual, but is inherent, and transmitted to offspring in just 

 the same way as other natural varieties. Of the causes which produce 

 such variation we know nothicg, but observation and experiment may 

 discover what circumstances favour its persistence and spreading in 

 some cases, and its disappearance or rarity in others. But if it be true 

 that, in some cases, melanism may be produced in nature by the action 

 of external conditions on the individual, while we know that this is not 

 so in other species, it is most important that we should have further 

 observation to enable us to distinguish between these different forms. 



The chief object of the present inquiry was to determine the 

 geographical range of melanism at present as a basis for future com- 

 parison, and to collect as much information as possible about its 

 spread within the memory of living observers. The extent of the 

 melanic area varies widely in different species ; some, e.g., M. strigilis, 

 are dimorphic over their whole range, but in such there may be evidence 

 that the dark form is becoming more frequent in some localities, and 

 tending to the exclusion of the light. In the majority of species the 

 dark form is commonest in one or more centres, and becomes scarcer 

 as the distance from the centre is increased. The melanic area may 

 be restricted, as in H. abruptaria and A. psi, or very large as in 

 X. pohjodon, and at the present time in A. betularia. In some species 

 melanism exists in certain areas, but shows no tendency to spread ; 

 in others it has increased its range rapidly within a few years. When 

 such spreading has occurred, it is almost always irregular ; in some 

 directions it has progressed quickly to great distances, while in others 

 the type remains predominant quite near the melanic centre. 

 Several examples of this are given in the summary of evidence 

 received, e.g., the black betularia first appeared near Manchester ; it 

 has spread over Lancashire, Yorkshire, and the northern Midlands, 

 and has now been taken in all the eastern counties, and is not rare 

 in London, but in the south and southwestern counties, and in 

 Scotland, it is still rare or absent. Perhaps even more remarkable is 

 its spread on the Continent, where, beginning apparently from 

 Belgium, it travelled up the Rhine and has now reached Saxony, 

 Silesia, and Berlin. The black variety of this species appeared in 

 Belgium before it was known in the eastern counties or in London, 

 and it is possible that it arose there independently, but it is, perhaps, 

 more likely that it was imported from England and established itself 

 there. The same problem is presented by P. pnlosaria, of which a 

 black variety is found in the Swansea district, which is said not to 

 differ from that taken in Yorkshire ; here, again, it may have been 

 artificially introduced. This species also is very capricious in the 

 spreading of its melanic variety ; the black form has rapidly become 



