MBILANISM AND MELANOCHROISM. 323 



appears to be convincing proof of the theory that bands are 

 formed by the union of transverse lines in certain parts of the 

 wing, coupled with the suppression of the transverse lines in 

 the other parts, in the same way that the lines forming a 

 central band are frequently suppressed on the inner margin, 

 leaving only a dark costal patch. Now, if we look on the pale 

 forms, at present typical with us, as the type, there is no 

 doubt that the dark forms must be looked upon as reversions, 

 and, on the contrary, if, as I assume, these dark forms have 

 really never been actually modified, but represent the original 

 form, then we must look on the banded forms, those with a costal 

 patch, and forms without dark markings, as all steps in one 

 gradual line of development, of which the latter are the higher 

 forms. I believe at any rate, that it is impossible to separate 

 melanisms due to reversion, from melanisms due apparently 

 to development, as some instances may simply be the exact 

 converse of others. 



Heredity was one of the points which I suggested should not 

 be overlooked in considering the production of melanism, but 

 there is yet another cause of melanism that should be men- 

 tioned, and that is disease. There is no doubt that disease, 

 especially disease brought about by continued interbreeding 

 and the consequent gradual weakening of the race, does pro- 

 duce melanism in certain species. In the Entomologist s Record^ 

 vol. i., pp. 236, 237, I have just touched on the subject, and 

 instanced how often crippled specimens, the result probably of 

 diseased or weak larvae, are darkened in colour, and specially 

 instanced a large brood of inbred Arctia nienthastri which were 

 very variable in markings, and which all showed a melanic 

 tendency. But attention has only just been drawn to the 

 subject, and there is no doubt that more facts will soon be 

 forthcoming. I have, at the present moment, in my possession 

 a very long and varied series of Ciispidia alni bred by Dr. 

 Chapman. This species, he informs me, he has bred and 

 inbred for some years. The specimens were all at first quite 

 normal in colour and markings, and whilst this was so, the 

 progeny were interbred without difficulty. This year, the 

 brood produced scarcely any typically coloured specimens ; 

 almost all were varieties, and much darker (some very much) 

 than usual, but the moths failed to produce fertile ova, and the 

 brood died out. Here, it seems almost certain, that the darken- 

 ing was due to disease, brought about by " in-and-in breeding." 

 I find also a remark by Mr. J. C. Warburg which runs as 



