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 _betularia_ 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 fore-wings 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 intermediate, but it almost certainly did. In certain places as for instance in Belgium, there is evidence that intermediates have at various times been fairly abundant, but they have never become common, nor are they known to exist in the absence of _doubledayaria_. When the black variety and the light type breed together they do not usually have intermediates among their offspring, and the evidence is consistent with the view that the black is a complete dominant. The same is probably true of _Tephrosia consonaria_. In some of the other species we know that the darkest forms did not appear first. For example in _Phigalia pilosaria_ and _Boarmia rhomboidaria_ dark forms existed and are believed to have increased in number before the darkest made its appearance. _Hybernia progemmaria_ is said to have become darker gradually both in Cheshire and in the West Riding, and a uniformly smoky variety appeared in South Yorkshire less than 45 years ago which has spread to neighbouring counties. The dark medium has become the commonest form in Huddersfield district, where the very dark variety is now about 20 per cent. of the population, though the light form is still common.