498 GEOGRAPHICAL DISTRIBUTION OF INSECTS. 
torial genera accompany the date, the sugar-cane, the 
indigo and banana a . The idea is very ingenious, and, 
under certain limitations, supplies a useful and certain 
criterion. For though none of these plants are univer- 
sal in isothermal parallels of latitude ; yet, as plants are 
more conspicuous than insects, the Entomologist, fur- 
nished with an index of this kind, may by it be di- 
rected in his researches for them ; and in all countries 
in which there is a material change of the climate, as 
in France, there will be a proportional change in the 
vegetable accompanied by one in the insect produc- 
tions. 
ii. In considering the range of insects I shall first ad- 
vert to that of individual species. At the extreme limits 
of phanerogamous vegetation we find a species of hum- 
ble-bee (Bombus arcticus), which, though it is not known 
to leave the Arctic circle, has a very extensive range to 
the westward of the meridian of Greenwich, having been 
traced from Greenland to Melville Island ; while to the 
eastward of that meridian it has not been met with. In 
Lapland its place appears to be occupied by B. alpinus 
and lapponicus, with the former of which, though quite 
distinct, it was confounded by O. Fabricius ; but whe- 
ther these range further eastward of that meridian has 
not been ascertained. From its being found in the 
Lapland Alps b , it ra^ r be conjectured that B. alpinus 
ranges as high on this side as B. arcticus on the other, 
and may perhaps be found in Nova Zembla. Some 
species that have been taken in Arctic regions are not 
confined to them. Of this kind is Dytiscus marginalis, 
■ Geographic &c. 20—. b See above, p. 494. 
