From the data as to the relative proportions of the polymorphic females of _P. polytes_ during the past and at present, and from the behaviour of their different forms in breeding, the following conclusions only can be drawn. Either natural selection, from the point of view of mimicry, is non-existent for this species in Ceylon, or else it is so slight as to be unable in half a {101} century to produce an appreciable diminution in the proportion of non-mimetic females. For even if the mimetic resemblance brings about but the survival of one additional protected form in 100 as compared with the unprotected, this means a marked diminution in the proportion of M females in 50 years--a diminution such as there are no grounds for supposing to have taken place. It has been argued that in populations exhibiting Mendelian heredity even a relatively low selection rate must bring about a rapid change in the constitution of a mixed population. Have we any grounds for supposing that populations of this sort can undergo such rapid changes? In cases where mimetic resemblances are involved we have no examples of the sort. But some interesting evidence as to the rate at which a population may change is to be gathered from the study of melanism in certain moths. It is well known that in some parts of England the common peppered moth, _Amphidasys betularia_ has been almost entirely supplanted by the darker melanic form _doubledayaria_. It first made its appearance near Manchester in 1850, and from that centre has been gradually spreading over northern England, the Midlands, and the south-eastern counties. At Huddersfield, for instance, fifty years ago only the type form _betularia_ existed; to-day there is nothing but _doubledayaria_. In Lancashire and Cheshire the type is now rare. On the continent, too, there is the same story to be told. The melanic form first appeared in Rhenish {102} Prussia in 1888; to-day it is much more abundant than the older type. There, too, it is spreading eastwards and southwards to Thuringia, to Saxony, to Silesia. What advantage this new dark form has over the older one we do not know[52]. Some advantage, however, it must have, otherwise it could hardly supplant _betularia_ in the way that it is doing. From our present standpoint two things are of interest in the case of the peppered moth--the rapidity with which the change in the nature of the population has taken place, and the fact that the two forms exhibit Mendelian heredity, _doubledayaria_ being dominant and _betularia_ recessive[53]. Moreover, mixed broods have been reared from wild females of both sorts, and so far as is known the two forms breed freely together where they co-exist. This case of the peppered moth shews how swiftly a change may come over a species[54]. It is not at all improbable that the establishing of a new variety at the expense of an older one in a relatively short space of time is continually going on,