, while
even some genera characteristic of the southern hemisphere appear to have
been originally derived from Europe. Thus Eucalyptus and Metrosideros have
been determined by Dr. Ettingshausen from their fruits in the Eocene beds
of Sheppey, while Pimelea, Leptomeria and four genera of Proteaceae have
been recognised by Professor Heer in the Miocene of Switzerland; and the
former writer has detected fifty-five Australian forms in the Eocene plant
beds of Haering (? Belgium).[140] Then we have such peculiar genera {519}
as Pachychladon and Notothlaspi of New Zealand said to have affinities with
Arctic plants, while Stilbocarpa--another peculiar New Zealand genus--has
its nearest allies in the Himalayan and Chinese Aralias. Following these
are a whole host of very distinct species of northern genera which may date
back to any part of the Tertiary period, and which occur in every south
temperate land. Then we have closely allied representative species of
European or Arctic plants; and, lastly, a number of identical species,--and
these two classes are probably due entirely to the action of the last great
glacial epoch, whose long continuance, and the repeated fluctuations of
climate with which it commenced and terminated, rendered it an agent of
sufficient power to have brought about this result.

Here, then, we have that constant or constantly recurrent process of
dispersal acting throughout long periods with varying power--that
"continuous current of vegetation" as it has been termed, which the facts
demand; and the extraordinary phenomenon of the species and genera of
European and even of Arctic plants being represented abundantly in South
America, Australia, and New Zealand, thus adds another to the long series
of phenomena which are rendered intelligible by frequent alternations of
warmer and colder climates in either hemisphere, culminating, at long
intervals and in favourable situations, in actual glacial epochs.

_Geological Changes as Aiding Migration._--It will be well also to notice
here, that there is another aid to dispersion dependent upon the changes
effected by denudation during the long periods included in the duration of
the species and genera of plants. A considerable number of {520} the plants
of the Miocene period of Europe were so much like existing species that
although they have generally received fresh names they may well have been
identical; and a large proportion of the vegetation during the whole