INTRODUCTION. 



In the introduction to Vol. I of this work, I referred shortly to 

 the extent and probable causes of variation in the NOCTILZE, and do not 

 now propose to travel further in the same direction, but rather to ex- 

 tend the view over a few particular points not then dealt with. There 

 are, however, one or two suggestions previously mentioned which have 

 received valuable confirmation during the last few months, and to these 

 I will briefly refer. 



In the introduction to Vol. I., pp. xv-xvi., I referred cursorily to 

 disease, as being a potent factor in producing variation, generally in a 

 direction tending towards melanism. Mr. Merrifield, to whom we are 

 greatly indebted for some valuable experiments relating to the influ- 

 ence of temperature on pupse in producing colour variation in the 

 resultant imagines, has made further experiments this year, one of the 

 species operated on being Vanessa urticce. I was fortunate in being 

 present at the meeting of the London Entomological Society when 

 these specimens were exhibited, and my idea that the darkening pro- 

 duced in Mr. Merrifield's experiments was due to some form of disease 

 received the fullest confirmation. The specimens exhibited did, 

 certainly, display a fair amount of variation, and those which had been 

 exposed artificially to the greatest and most continued cold were the 

 darkest, but, at the same time, with scarcely a single exception, were 

 all more or less deformed. The result of the application of artificial 

 cold here becomes self-evident. The suspense of the vital functions, 

 at a time just previous to emergence, when they should be most active, 

 undoubtedly affects injuriously the constitution of the pupse, the re- 

 sulting imagines, if any, being deformed, ill-developed as to scale 

 structure, and may become rather paler, or darker according as the 

 retrogression naturally tends towards a paler . or darker coloration. 

 The influence of excessive artificial cold, in producing variation is now 

 comparatively clear. It appears to be entirely indirect, and simply 

 acts by producing a diseased condition in the pupa, and a resultant 

 imago more or less deformed. It also appears to prevent the proper 

 formation of pigment and hence produces, as it were, an excess of 

 non-pigmental scales at the expense of the normal ones. The general 

 obsolescence of characteristic colour and markings, in those species 

 which are essentially Arctic, is now explained, as we see that the 

 vital functions are less active and therefore not so capable of develop- 

 ing scale and pigment. In some butterflies, the resulting influence 

 differs from that in some moths, as the influence of " natural selection " 

 is generally so essentially diverse in the two groups ; and the result 

 of retrogression is also different. As an illustration of what I mean, 

 it may be safely assumed, that the red-brown of polychloros and the 

 bright red of urticce are both equally developed through yellow. Both 

 are almost equally soluble as pigments, passing through yellow to 



