The class Hexapoda has a world-wide range, and so have most of its
component orders. The Aptera have perhaps the most extensive
distribution of all animals, being found in Franz Josef Land and South
Victoria Land, on the snows of Alpine glaciers, and in the depths of the
most extensive caves. Most of the families and a large proportion of the
genera of insects are exceedingly widespread, but a study of the genera
and species in any of the more important families shows that faunas can
be distinguished whose headquarters agree fairly with the regions that
have been proposed to express the distribution of the higher
vertebrates. Many insects, however, can readily extend their range, and
a careful study of their distribution leads us to discriminate between
faunas rather than definitely to map regions. A large and dominant
Holoarctic fauna, with numerous subdivisions, ranges over the great
northern continents, and is characterized by the abundance of certain
families like the _Carabidae_ and _Staphylinidae_ among the Coleoptera
and the _Tenthredinidae_ among the Hymenoptera. The southern territory
held by this fauna is invaded by genera and species distinctly tropical.
Oriental types range far northwards into China and Japan. Ethiopian
forms invade the Mediterranean area. Neotropical and distinctively
Sonoran insects mingle with members of the Holoarctic fauna across a
wide "transition zone" in North America. "Wallace's line" dividing the
Indo-Malayan and Austro-Malayan sub-regions is frequently transgressed
in the range of Malayan insects. The Australian fauna is rich in
characteristic and peculiar genera, and New Zealand, while possessing
some remarkable insects of its own, lacks entirely several families with
an almost world-wide range--for example, the _Notodontidae_,
_Lasiocampidae_, and other families of Lepidoptera. Interesting
relationships between the Ethiopian and Oriental, the Neotropical and
West African, the Patagonian and New Zealand faunas suggest great
changes in the distribution of land and water, and throw doubt on the
doctrine of the permanence of continental areas and oceanic basins.
Holoarctic types reappear on the Andes and in South Africa, and even in
New Zealand. The study of the Hexapoda of oceanic islands is full of
interest. After the determination of a number of cosmopolitan insects
that may well have been artificially introduced, there remains a large
proportion of endemic species--sometimes referable to distinct
genera--which suggest a high antiquity for the truly insular faunas.