VARIATION AND LOCALITY 



139 



mediate, 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 be- 

 come 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. 



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 ele- 

 ment 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 individuals have gradually superseded the normal type more or 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 defi- 

 nite 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 dis- 



