Humboldt noticed them up
to heights of 15,000 and 18,000 feet in South America, and Mr. Albert
Mueller has collected many interesting cases of the same character in
Europe.[173] A moth (Plusia gamma) has been found on the summit of Mont
Blanc; small hymenoptera and moths have been seen on the Pyrenees at a
height of 11,000 feet, while numerous flies and beetles, some of
considerable size, have been caught on the glaciers and snow-fields of
various parts of the Alps. Upward currents of air, whirlwinds and
tornadoes, occur in all parts of the world, and large numbers of insects
are thus carried up into the higher regions of the atmosphere, where
they are liable to be caught by strong winds, and thus conveyed enormous
distances over seas or continents. With such powerful means of
dispersal the distribution of insects over the entire globe, and their
presence in the most remote oceanic islands, offer no difficulties.
_The Dispersal of Plants._
The dispersal of seeds is effected in a greater variety of ways than are
available in the case of any animals.
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