PROGRESSIVE MELANISM IN LEPIDOPTERA. 249 



perhaps, of great importance, is that in some species there is a sharp 

 distinction between melanic and typical specimens, intermediates being 

 very rare ; in others there is a gradual progressive darkening from the 

 lightest to the darkest forms. Examples of the discontinuous class are 

 Amphidasys betnlaria, Hemerophila ahruptaria, Venusia cambrica, 

 Acidalia aversata ; of the continuous, Odontopera bidentata, Xylophasia 

 polyodon, Diurnaea fagella. But, in some cases, where the variation is 

 apparently continuous, breeding experiments may reveal undoubted 

 discontinuity among the offspring of the same parents. This was well 

 illustrated by the beautiful series of Triphaena comes [orbona) exhibited 

 by Mr. Bacot at the meeeting of the City of London Entomological 

 Society on November 7th, 1905. A casual observer of his insects 

 might say that, in the families which contained both light and dark 

 specimens, every grade existed between the lightest and darkest, but 

 on closer inspection it became at once manifest that a perfectly definite 

 line could be drawn. There was no doubt at which point in the series 

 the separation came ; above it, all the moths were typical, below it, all 

 melanic, but the intensity of the melanism varied greatly, from a slight 

 suffusion to almost complete blackness. This suggests that melanism 

 may be a discontinuous character, although the intensity with which 

 the darkening appears in individuals fluctuates ; a sharp line separates 

 melanic from non-melanic, but the depth of melanism varies continu- 

 ously among the individuals affected. In some species, e.g., A. betularia, 

 the melanism, when present, is nearly always complete, but in these 

 also rare intermediates occur, which may probably be regarded as cases 

 in which the darkening is present, but less intense than usual. Such 

 specimens are distinguishable at a glance from dark examples of the 

 type in which the markings are definite, but more than usually exten- 

 sive ; both the type and the melanic form exhibit fluctuating variability, 

 but a given insect can almost always be referred without hesitation to 

 one or the other. On the other hand, in such species as X. polyodon, 

 it is difficult to draw any line, or to be confident that any discontinuity 

 exists, and to elucidate such cases further observation and experiment 

 would be of great value, for it is possible that they belong to an 

 independent category of variation having different causes. 



Some of the breeding experiments which have been made with 

 melanic species indicate that melanism is inherited in accordance with 

 Mendel's law. This appears to be the case in T. comes (Bacot) and 

 H. abruptaria (Harris, Hamling, although in Mr. Harris' experiments 

 the numbers diverge widely from Mendelian expectation). In other 

 species, e.g., A. betularia, it will probably be found that the same form 

 of inheritance prevails, the melanic form being dominant in each case 

 over the type. In other cases the inheritance is not Mendelian, e.g., 

 in A. nehulosa the darkest form crossed with the grey gave inter- 

 mediates in addition to grey and black, and grey parents may throw 

 a proportion of black offspring. 



It has long been known that variations in temperature, moisture, 

 and other conditions may cause differences in the colour and pattern of 

 lepidoptera, but such varieties disappear in the offspring when they are 

 reared under normal conditions. In some cases where varieties have 

 been produced by extremes of temperature, a small part of their off- 

 spring has also been abnormal (Fischer), but nothing like the 

 inheritance found in typical melanic cases has been observed. Grafin 



