308 THE PLACE OF MIMICRY 



chalk, dark when found upon peat. Naturalists who 

 have had much experience of the species in its native 

 haunts assert that its habits are such as to render these 

 tints highly protective. Careful experiments 1 have 

 proved that when nearly full-fed larvae are transferred 

 from chalk to a very dark background, and kept there 

 during the whole of the pupal period, the tints of the 

 resulting moths are in no way affected. Unless the 

 imaginal colours are determined by stimuli applied still 

 earlier in the larval life — upon the whole an improbable 

 conclusion — we can only suppose that the local colour- 

 harmony has been produced by the gradual destruction 

 through many generations of the darker forms on chalk 

 and the paler ones, upon peat, &c. 



1 2. The Recent Progressive Darkening of many Species 

 of Moths in the Lancashire and Yorkshire District.-^A 

 great deal of evidence has been brought forward to show 

 that many moths in the neighbourhood of great centres 

 of population, and also in distant areas affected by their 

 smoke, have become very different in appearance from 

 what they were during the lives of the last generation of 

 naturalists and even within the memory of many still 

 living. The latest records on the subject are those of 

 Mr. G. T. Porritt, 2 who does not himself accept the con- 

 clusion that the change has been caused by Natural 

 Selection. The smoke from the manufacturing districts 

 of Yorkshire, driven by the prevalent S.W. wind, has 

 killed the lichen and rendered the tree trunks and 

 branches uniformly dark over a wide strip of country. 

 Their appearance has become utterly different, and the 

 pale variegated tints formerly borne by the moths in 

 question would now stand out with startling distinctness 

 upon them. It is of the utmost importance to gain the 

 fullest knowledge of the character of the struggle for 

 existence in these species, and to test the interpretation 

 founded on Natural Selection in every possible way; 

 but the facts as they stand seem to raise an over- 

 whelming probability in favour of this explanation, which 



1 Trans. Ent. Soc, Lond., 1892, pp. 453-8. 



2 British Association, York, 1906, Report, p. 316. 



