MELANOCHROISM IN BRITISH LEPIDOPTERA. 143 



to criticise, and this was managed so long as I kept within the confines 

 presented by an elementary, physical geography text-book, but when 

 I get beyond these limits, the pajjer becomes '' dim and obscure " to 

 Mr. Kobson. But even in physical geogi'aphy Mr. Eobson is behind 

 the times, for he talks of the increased humidity caused by the Gulf 

 Stream as " assumed." This is a matter of fact, not assumption. His 

 knowledge, too, of the distribution of melanic forms in Britain is shaky, 

 for he speaks of my " attempting to show that these are the districts 

 where melanism most prevails." This, again, is a matter of fact, and 

 not of attempt. 



If Mr. Kobson had understood what I had written on the subject, 

 he would not have given me credit for restricting nature's action to such 

 narrow lines as he does. The breadth of my views, and the considera- 

 tion of possible contingencies, seem to be the great blot in the paper, 

 according to my critic. I pointed out broadly, that where there was 

 excessive moisture, or excessive smoke, or both combined, rock-resting, 

 and tree-frequenting species, became melanic. This form of melanism, I 

 ■considered due to two things : (1) Moisture (or smoke, or both com- 

 bined), which, in some way, acted physiologically on the larva?, aifecting 

 them in such a manner, as to make the sjjecies vary in a general way ; 

 (2) "Natural selection," which selected the dark forms produced by the 

 general variation, and formed a race, suitable by constitution, to its en- 

 vironment. I still supi^ose " moisture " is the great factor in inducing 

 •or producing the primary physical change in the larvaj, in Britain 

 (although many other causes may have a similar effect elsewhere), re- 

 sulting in this instance, in a melanotic development, and nothing as yet 

 has been urged against this opinion. On the contrary, it seems to have 

 been very generally accepted. With regard to this, I might perhaps go 

 a stej:) further, and point out, that although the final result of the action 

 of moisture and smoke is much the same, the effect has been rather 

 different. Moisture has acted slowly through ages on the constitution 

 of the larva, and has thus brought about a certain amount of general 

 variation; the secondary action of moisture, due to " natural selection," 

 has likewise been slow, but always in the direction of melanism, by 

 selecting the dark forms (or races) most protected by darker environ- 

 ment, brought about by the moisture, as shown by Dr. Chapman and 

 myself. The action of smoke has been rapid, because it is practically 

 a new phase of environment. It also, as an unusual factor, has acted 

 on the constitution of the larva, and produced change and variation. 

 But it also, similarly to moisture, acts in a secondary way, and also in 

 the direction of darkening objects, but its area is more generally limited 

 to tree-trunks, fences, walls, iSzc, in the comparative near vicinity of its 

 production. Indirectly, therefore, the force of " natural selection " has 

 again cleared out the paler, and protected the darker forms. Hence we 

 see, that the artificial, so to speak, environment, has with " natural 

 selection " produced a much more rapid, and sometimes, esjjecially when 

 aided by rain, a more complete change in a shorter jJeriod, than has the 

 natural environment in an incomparabl}^ longer time. 



Mr. Kobson further -wi-ites : — " In our fens and bogs, our wet moors 

 and mosses, we would surely find some evidence in support of this theory, 

 if it were true." Has Mr. Kobson so little knowledge of the insects 

 from these localities, or, so little material in his collection, to suppose 

 that such localities do not give numbers of insects which support this 



