308 THE CAUSES WHICH 



Another actual or obsolete line of migration of the Scottish 

 moth and butterfly fauna southwards may be found in the 

 elevated mountain chains of the Lake District and Wales ; but 

 few, however, in their distribution pass into the latter district, 

 which has, again, many species peculiar to it. Mr. Birchall some 

 time back remarked the similarity of the northern lepidopterous 

 fauna of Ireland to that of Scotland. Here we have not alone 

 the Large Heath Butterfly (C. Davus) in the Donegal Moun- 

 tains, but many of the more northern British Moths occur, such 

 as Acronyctcb menyantliidis, E^unda nigra, Plmia inter rogationis, 

 PMhalapteryx lapidata, and Peronea onaccana ; and although 

 present flux and migration have not been noticed to or from 

 Caledonia, they are more than probable ; since Fair Head in Ire- 

 land is only fifteen miles distant from the red rocks of Cantyre, 

 and Belfast Bay only twenty miles from Port Patrick ; a less 

 water interval in fact than that presented by the boisterous 

 Straits of Dover. As regards Europe, the Mediterranean basin is 

 more or less regularly traversed in April and May by swarms 

 of the Cabbage Butterfly and Painted Lady, which on arriving 

 in France oviposite. These are considered as the avant-courenrs 

 of the quails. Night-fliers uncertain in appearance, and which do 

 not regularly breed in Europe, as the Hawk Moths, CJmrocampa 

 nerii and celerio, or Deilephila lineata, similarly arrive, sometimes 

 in broad day. Since the insects of Eastern Siberia are considered 

 to have special affinity with those of Western America, Behring 

 Strait may be regarded as another hypothetical line of distri- 

 bution ; and many European butterflies straggle from the Alps 

 by the elevated Asian chains into Northern India. 



Insects of other orders, with wings more rigid or weaker, 

 as a rule float on the gale. Prominent among such are the 

 locusts or larger Orthoptera, whose disastrous name we, from 

 our childhood, associate with seasons of southern drought, whirl- 

 winds, hailstorms, earthquakes, plagues, famine, volcanic erup- 

 tions, and other slumbering powers that shook the foundations 

 of the globe in its earliest ages, or of society in its earliest 

 history. These species, although by their strong fore-wings 

 and distinct discoidal cell endowed with some powers of flight, 

 nevertheless do not appear capable of " warping on the eastern 

 wind,^' but rather allow themselves to be swept along on the 



