forms--is held to be sufficient. In this particular case, however,
we have some very remarkable evidence of the fact of their non-adaptation.
The intercourse between New Zealand and Europe has been the means of
introducing a host of common European plants,--more than 150 in number, as
enumerated at the end of the second volume of the _Handbook_; yet, although
the intercourse with Australia has probably been greater, only two or three
Australian plants have similarly established themselves. More remarkable
still, Sir Joseph Hooker states: {508} "I am informed that the late Mr.
Bidwell habitually scattered Australian seeds during his extensive travels
in New Zealand." We may be pretty sure that seeds of such excessively
common and characteristic groups as _Acacia_ and _Eucalyptus_ would be
among those so scattered, yet we have no record of any plants of these or
other peculiar Australian genera ever having been found wild, still less of
their having spread and taken possession of the soil in the way that many
European plants have done. We are, then, entitled to conclude that the
plants above referred to have not established themselves in New Zealand
(although their seeds may have reached it) because they could not
successfully compete with the indigenous flora which was already well
established and better adapted to the conditions of climate and of the
organic environment. This explanation is so perfectly in accordance with a
large body of well-known facts, including that which is known to every
one--how few of our oldest and hardiest garden plants ever run wild--that
the objection above stated will, I feel convinced, have no real weight with
any naturalists who have paid attention to this class of questions.

* * * * *

{509}

CHAPTER XXIII

ON THE ARCTIC ELEMENT IN SOUTH TEMPERATE FLORAS

European Species and Genera of Plants in the Southern
Hemisphere--Aggressive Power of the Scandinavian Flora--Means by which
Plants have Migrated from North to South--Newly moved Soil as Affording
Temporary Stations to Migrating Plants--Elevation and Depression of the
Snow-line as Aiding the Migration of Plants--Changes of Climate
Favourable to Migration--The Migration from North to South has been
long going on--Geological Changes as Aiding Migration--Proofs of
Migration by way of the Andes--Proofs of Migration by way of the
Himalayas and Southern Asia--Proofs of