i48 [December. 



the melanochroism. Melanochroism once set up and advantages found 

 to accrue from the possession of it, then natural selection comes into 

 play, and eventually, perhaps, melanochroic races are established. In 

 other cases, where only a proportion of the individuals of a species 

 are melanochroic, the special conditions which made melanochroism 

 an advantage to its possessor, may have ceased to exist, and con- 

 sequently circumstances permit of a reversion to ancestral forms ; 

 or the melanochroic may be the ancestral form, and the dark 

 individuals are reverting. In like manner leucochroism may be an 

 advantage to its possessor, and have been similarly developed by natural 

 selection. 



That there is an exciting cause for both forms I am persuaded, 

 because we find that species which are sufficiently common for us to 

 observe year after year in abundance, are found to be much more 

 subject to variation in some years than in others ; and if (presumably) 

 the meteorological differences of one year form another cause, in a 

 single locality of a varying amount of variation in species, we may 

 reasonably conclude that the meteorological differences between one 

 locality and another, continued year after year, will tend to variation 

 in different directions in the individuals of a species common to both. 

 Then if in one locality where the struggle for existence is greater, a 

 peculiar variation is found to carry advantages with it, natural selec- 

 tion steps in and does its work, and, if the advantage is very great, 

 may eventually result in that particular variety supplanting all others. 

 If the advantages were not so great, the particular variety would not 

 be so peculiarly favoured ; and if there was no special advantage (but 

 still no disadvantage), then the variety would only be on an equal 

 footing with the other forms, and individuals of that character would 

 vary in number from year to year according as the meteorological 

 conditions (presumably the exciting cause) varied, with a certain 

 percentage for heredity. 



By this theory, local varieties and aberrations (both melanochroic 

 and otherwise) may perhaps be accounted for. For example, let us 

 take the ab. Jirthlandica of Hepialus humuli. This is a form of the 

 ^ in which the usual satiny white colour is frequently tinged with 

 yellow, and dark markings, as in the $ , are more or less apparent. 

 At the time this moth is on the wing there is scarcely any darkness 

 in Zetland, and consequently the hovering white moths must be A^ery 

 conspicuous and easily seen by gulls and other birds, which, I have 

 noticed in other localities, eagerly pursue them. But a (J of darker 



