84 THE ENTOMOLOGIST. 



" Coming to the question of archaic derivation the researches and experi- 

 ments of Weismann* and Edwardsf may be admitted to have established 

 a well-supported theory that in cases of seasonal dimorphism and poly- 

 morphism, one or other of the varieties produced is probably that which 

 has descended through the longest period from an ancestral form. Both 

 these authors have called attention to the fact that there is less disposition 

 to vary in the female sex, and if the females are truly more conservative 

 than the males, we should be inclined to look to the former sex as most 

 likely to indicate the typical coloration of an archaic race. 



" Mr. M'Lachlan| pictures the survivors of an Arctic fauna moving 

 northward as increasing temperature succeeded the glacial period, a portion 

 of them settling on the tops of high mountains, stragglers reaching the 

 home of their ancestors, and becoming the progenitors of our present 

 Arctic species ; but he fails to suggest that the condition of existence in 

 those two distinct settlements being approximately the same, a natural law 

 producing certain forms in the one place might be equally operative to 

 produce them in the other. 



" Admitting the extreme value and interest of these researches, and 

 allowing them their due weight in the collection of evidence upon the 

 general subject, we may remark that their authors in no way profess to 

 account for the primary cause of the melanic tendency, except in so far as 

 they would admit it to be traceable to the external influence of climatic 

 conditions. We may yet ask ourselves, What is the exact method by which 

 the pigments are acted upon, and what advantages, if any, do the insects 

 derive from the change ? ' 



On the subject of insular variations it is remarked : — 



" Mr. de Vismes Kane, in his address to the Barnsley Naturalists' 

 Society, § on the Variation of European Lepidoptera, properly pointed out 

 that ' all naturalists are agreed that the strongest developed variations are 

 cut off from intercommunication with the rest of their kind by mountains, 

 vast forests, the sea, or other natural barrier.' But although, as he says, 

 'isolation begets peculiarity,' I am unable to agree that this is the principal 

 cause of the aberrant coloration of the Shetland insects." 



Reasons are then given in favour of the hypothesis that the 

 darkened coloration of the Lepidoptera from high latitudes and 

 altitudes is an advantage to them, inasmuch as they are thus 



* ' Studies in the Theory of Descent,' vol. i., and ApiDendix; English edition, 

 t ' Butterflies of North America,' and ' Canadian Entomologist," vol. vii. 

 pp. 228—240; vol. ix. pp. 1-10, 51—55, 203—206. 



+ 'Journal of Linnean Society' (Zoology), 1878, xiv. p. 105. 

 § ' The Naturalist,' November, 1884, p. 82. 



