IXTRODUCTIOX. 



XXI 



of the second brood passing the winter in the pupa state. It often happens that these two 

 broods are so dissimilar that no one would imagine them to belong to the same species, unless 

 they had been bred. Among the most striking instances are the broods of Arascluiia Lcvana 

 among the butterflies, and those of the genus Selenia among the Gtoirietrce. Many species 

 which are usually single-brooded may develop a second brood in an unusually fine season, or 

 a double-brooded species may produce a third generation ; but, on the other hand, when the 

 season is unfavourable, a species which is usually double-brooded may appear only once. But 

 notwithstanding this, it alwaj'S happens that tlie perfect insect and the larva appear at the 

 usual time. Many species are double-brooded in the south, or in the plains, which are single- 

 brooded further north, or in the mountains. 



Distribution. — Lepidoptera are distributed over the whole surface of the globe, but not in 

 equal numbers. They are dependant upon vegetation, and are most numerous where the vegeta- 

 tion is most lu.xuriant or most varied. In Europe the latter is the case ; and the countries lying 

 around the great central mountain ranges possess by far the richest Lepidopterous fauna. On their 

 southern slopes we find most of the species belonging to the Mediterranean fauna ; on their 

 northern slopes most of the species of Central Europe ; and on their summits many of the 

 species which occur in the e.xtreme north of Europe, just as we find many northern plants on 

 the Alps, mingled with others peculiar to themselves. North and south of the Alps the 

 number of species rapidly diminishes, till it reaches its minimum on the shores of the Arctic 

 Ocean in one direction, and on the outskirts of the Sahara on the other. Even beautiful and 

 fertile Andalusia scarcely produces more butterflies than Sweden ; for the Alpine species of 

 Central Europe have nearly all disappeared in the former country, and many of our most 

 familiar insects are exclusively confined to the mountains, so far to the south. 



As regards the vertical distribution of Lepidoptera, five regions have been defined by the 

 Messrs. Speyer, in their work on the Geographical Distribution of German and Swiss Lepidoptera. 

 The first is the lower region, and is bounded by the limit above which the walnut-tree ceases 

 to grow. In Central Germany this is about 450 metres above the sea ; in the northern lime- 

 stone Alps, about 750 metres ; and in the Southern Alps, about 900 metres. Next to this is 

 the hill region, to the limit of the beech-tree, which reaches a height of 900 metres in Central 

 Germany, and 1,200 in the Alps. Next to this is the lower Alpine region, to the limit of the 

 pine-tree, Piitits Picea, from 900 to 1,350 metres in Germany, and from 1,200 to 1,800 metres 

 in the Alps. Above this is the upper Alpine region, extending above the region of the forests 

 to a height of from 2,100 to 2,250 metres; and, finally, the snow region, from this to the snow 

 line, and beyond. We give here a table, showing the comparative number of Central European 

 species of various families found in each of these regions respectively : — 



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