286 THE EVOLUTION THEORY 



some to the south to the Alps to escape in their heights from the increas- 

 ing warmth. There are many diurnal Lepidoptera which now belong 

 to both regions, and of these some have remained exactly alike, so that 

 the Arctic form cannot be distinguished from the Alpine form ; others 

 show slight differences, so that we can distinguish an Arctic and 

 an Alpine variety. To the former category belong, for instance, 

 Lyccmia donzslii and LjiccGna pheretes, Argynnis pales, Erebia manto, 

 and others ; to the second category belong, for instance, Lyccena 

 orbitulus, Prun., Lyccena optilete, Argynnis thore, and some species 

 of the genus Erebia. 



This cannot be an instance of the direct effect of general climatic 

 influences, for in that case all the nearly related species of a genus 

 would have varied or not varied ; nor have we to do with adaptations, for 

 the differences in marking are seen on the upper surfaces of the wing, 

 which do not exhibit protective colouring, at least in these Lepidoptera. 

 It can only have been the prevention of crossing that has fixed the 

 existing variational tendencies in the isolated colonies — variations 

 which Avould have been swamped and obliterated if there had been 

 constant crossing with all the rest of the members of the species. 



But there is another factor to be considered. Those Alpine 

 Lepidoptera, for instance, which have not remained exactly the same 

 in the far north, have formed local varieties in the rest of the area of 

 their distribution also, while species which have remained quite alike 

 in isolated regions, such as the Alps and the north, exhibit no aberra- 

 tions in other isolated regions, such as the Pyrenees, in Labrador, or 

 in the Altai. Thus one species must have had a tendency in the 

 Glacial period to form local varieties, and the other had not; and 

 I have already attempted to explain this on the hypothesis that the 

 former at the time of their migration and segregation into different 

 colonies were at a period of dominant variability, the latter at a 

 period of relatively great constancy. Leaving aside the question of 

 the causes of this phenomenon, we may take it as certain that there 

 are very variable and very constant species, and it is obvious that 

 colonies which are founded by a very variable species can hardly ever 

 remain exactly identical with the ancestral species ; and that several 

 of them will turn out differently, even granting that the conditions of 

 life be exactly the same, for no colony will contain all the variants of 

 the species in the same proportion, but at most only a few of them, 

 and the result of mingling these must ultimately result in the develop- 

 ment of a somewhat different constant form in each colonial area. 



If we were to try to imitate this ' amixia ' artificially we should 

 only require to take at random from the streets of a large town 



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