IN DEFENSIVE COLORATION 309 



was suggested many years ago by Mr. J. W. Tutt. 1 

 Under any circumstances the colour-harmony is not to 

 be explained, like that of many larvae and pupae, by 

 susceptibility of the individual at any time in its life to 

 the darkened tints of its environment. Eggs sent from 

 the smoky districts and reared in other parts of the 

 country produce the same dark moths. Furthermore, 

 the smoky districts act as centres, from which, by inter- 

 breeding, hereditary influence radiates into the surround- 

 ing areas ; and even so far off as Oxford, black forms of 

 some of the species are believed to be more prevalent 

 than formerly. It is of interest to note that the 

 changes which have occurred offer little support to the 

 hypothesis of evolution by mutation. Owing entirely 

 to artificial conditions, a sudden and rapid change has 

 occurred in the tints of the environment. In nature we 

 should rarely meet with such rapid and sudden changes. 

 In one species, Amphidasys betularia, a black 'mutation', 

 the form doubledayaria, existed as a rarity before the 

 darkening of the environment. Doubledayaria has now 

 replaced the typical form in the districts we are con- 

 sidering. But Mr. Porritt tells me that such a sudden 

 change is quite the exception among the numerous 

 species which have darkened. In the great majority, 

 a long series of intermediate varieties can still be found, 



1 Entomologist's Record, vol. i, 1 890-1. The following passage is 

 quoted from p. 56 : — ' The vast quantity of smoke, gases, fumes, &c, in 

 manufacturing towns, brought down by rain, is scarcely credible, and it 

 is from these impurities I consider the permanent darkening comes. 

 When the water evaporates, the solid matter is left behind, and as a 

 result the impurities are left to darken the surfaces to which they have 

 been carried by the rain-water. The theories of " natural selection " and 

 " protection " now apply in their fullest sense, the insects become 

 darkened, " hereditary tendency " perpetuating and intensifying the 

 melanism. I believe from this (and it appears to be a fair deduction), 

 that Lancashire and Yorkshire melanism is the result of the combined 

 action of the " smoke," &c, plm humidity, and that the intensity of York- 

 shire and Lancashire melanism produced by humidity and smoke, is 

 intensified by " natural selection " and " hereditary tendency ".' The only 

 words I should criticize in this passage are those which suggest that the 

 humidity and smoke have had any effect apart from that due to Natural 

 Selection. 



