Problems of genetics / by William Bateson.

  • Bateson, William, 1861-1926.
Date:
1913
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Problems of genetics / by William Bateson. Source: Wellcome Collection.

    160/290 (page 138)
    138 PROBLEMS OF GENETICS grey, but that of consonaría is a full black, and nothing like either has been found anywhere else. These examples are all taken from the Geometridae but others, though of a less conspicuous kind, could be given from the Noctuidae or the Micro-Lepidoptera. Acronycta psi, for instance, has a suffused form which is believed to be becoming more frequent in the London district. Folia chi has two dark forms, olivácea, a yellowish grey with dark markings, and suffusa which is a darker, blackish-slate colour. Both occur in the North of England, sometimes together, sometimes separately, or mixed with the type and many intermediates. The distribution is peculiarly irregular. At Huddersfield, where the very dark form appeared suddenly about 1890, some 30 per cent, are said to be now dark and about 6-7 per cent, very dark, but at Saddleworth, 12 miles away, only the pale forms occur. Several questions of interest arise in regard to this evidence. This progressive Melanism has arisen in certain families only, and may be confined to certain species only, within those families. As in almost all other examples in which variation has been much observed, its incidence is capricious and specific. A collateral line of inquiry relates to the degree of discontinuity which the variation manifests. Here again there is no rule. Generally speaking, in A. betularia, to take the case most fully studied, the variation is discontinuous. Real intermediates between betu¬ laria and doubledayaria are in most localities absent or rare. The black spots of betularia may often be larger or more numerous than in the normal, but this variation has nothing to do with doubledayaria, and is not an intermediate stage towards it, though sometimes wrongly so described. Doubledayaria owes its characteristic appearance to a factor which blurs the surface of the wings with a layer of black. Sometimes this blurring is slighter than in the real doubledayaria, and these forms are real intermediates. Occasionally the forewings alone are thus blurred. These intermediates are clearly due to reduction-stages of the doubledayaria factor, and are related to it as a blue mouse is to a black, or a dutch rabbit to a self-colour. It cannot positively be asserted that the full doubledayaria existed before the inter-