culture long enough for sucli differentiation to gccur : it isj 

 however, very possible that such differentiation was already 

 established before the pea was taken into cultivation, and, in 

 any case, the pea obtains very complete segregation by its 

 method of fertilisation, so that segregation easily takes place 

 without any geographical separation or any special care, and 

 its results may appear more quickly than in forms that tend 

 to continual crossing. 



" There is a series of facts that are connected with this view 

 of factors, but do not necessarily support or v/eaken it. These 

 depend on what may be described as factors that are not 

 recessive in the Mendelian sense, but in a much wider and 

 deeper manner. The facts of melanism in Lepidoptera may 

 be used to illustrate this. 



" Early in the evolution of the Lepidoptera dark coloration 

 would be very useful as assisting inconspicuousness, either on 

 dark surfaces or amongst unlighted surroundings, and con- 

 sequently many species no doubt acquired factors for melanism 

 of one sort or another. When no longer useful the melanic 

 factor must often have died out, and one cannot guess how 

 often it may have been acquired and lost in any one Lepido- 

 pterous stirps. Melanism, however, crops up so often in 

 individuals, sporadically, and is usually heritable, that it 

 seems necessary to suppose that the factor for melanism has 

 been present for many generations, or possibly ages, without 

 declaring itself. Probably, however, sporadic examples have 

 occurred, unobserved and unrecorded, throughout the whole 

 period. Many other variations in Lepidoptera that are found 

 at rare intervals are due to similarly receded factors — 

 ' buried ' might describe them, perhaps, better than receded. 



" The Tephrosiae and allied Boarmiinae, which have, in so 

 many species, developed melanism in our manufacturing 

 districts, have given rise to an idea that must be erroneous — 

 viz. that m^elanism has under our eyes originated de fiovo in 

 each of these species. It is much more probable that these 

 species have assumed and dropped the melanic facies many 

 times, back perhaps to the time of their common ancestor, who 

 possibly did so. They are a group that have the habit of 

 resting on tree-trunks and rocks, and the wetness or dryness 



