Taking the evidence together we find it consistent with the view that dark forms have appeared sporadically, in some species the very dark appearing first and intermediates later, in others the moderately dark came first and the darkest later in time. It is practically certain that the change has in general come about not by a gradual change supervening on the population at large, but by the sporadic appearance of dark specimens as a new element in the population, and strains derived from these dark individuals have gradually superseded the normal type more or less completely. If it could be shown that these melanic novelties had a definite advantage in the struggle for existence they would provide an instance of evolution proceeding much in the way which Darwin contemplated. The whole process would differ from that conceived by him as the normal method of evolution only in so far as the change has come about with great rapidity and in some instances largely by the appearance and success of discontinuous varieties. The question, however, must be asked whether the dark form can reasonably be supposed to have an advantage by reason of their darkness. Some naturalists believe that the darkness of the colours does thus definitely contribute to their protection by making the insects less conspicuous and thus more likely to escape the search of birds. In support of this view it may be pointed out that it is in the manufacturing districts of Lancashire and Yorkshire, and again in the London area that the melanics have attained their greatest development. Consistently with this argument also, it is in the neighbourhood of Crefeld and Essen, the black country of Germany, that they have chiefly established themselves on the Continent, and _Phigalia pilosaria_ in the black form is now at home in South Wales. Thus superficially regarded, the evidence looks rather strong, but it is difficult to apply the reasoning in detail. We have first the difficulty that the black form of _betularia_ for instance has established itself in thoroughly rural districts, notably near King's Lynn in Norfolk, and in the neighbourhood of Kendal and Windermere. The black form of _consonaria_ and the dark _consortaria_ appeared in a wood near Maidstone, far from town smoke, and the black _rhomboidaria_ was first found at Norwich, which, as towns go, is clean. Then again the spread of the melanics is very irregular and unaccountable. The black _pilosaria_ is found both in the West Riding and in the Swansea district, but not yet elsewhere. It rapidly increased at Huddersfield, but made no noticeable progress at Sheffield though recorded there for ten years. It is also a remarkable fact that no similar melanic development has been observed in America, and, so far as I am aware, comparable melanic varieties have not appeared on the European continent except in the case of the few sorts which possibly may have come from England.