important point
is, probably, that these plants are generally, if not always,
wind-fertilised, and are thus independent of any peculiar insects, which
might be wanting in the new country.

_Why Easily-Dispersed Plants have often Restricted Ranges._--This last
consideration throws light on a very curious point, which has been noted as
a difficulty by Sir Joseph Hooker, that plants which have most clear and
decided powers of dispersal by wind or other means, have _not_ generally
the widest specific range; and he instances the small number of Compositae
common to New Zealand and Australia. But in all these cases it will, I
think, be found that although the _species_ have not a wide range the
_genera_ often have. In New Zealand, for instance, the Compositae are very
abundant, there being no less than 167 species, almost all belonging to
Australian genera, yet only about one-sixteenth of the whole are identical
in the two countries. The explanation of this is not difficult. Owing to
their great powers of dispersal, the Australian Compositae reached New
Zealand at a very remote epoch, and such as were adapted to the climate and
the means of fertilisation established themselves; but being highly
organised plants with great flexibility of organisation, they soon became
modified in accordance with the new conditions, producing many special
forms in different localities; and these, spreading widely, soon took
possession of all suitable stations. Henceforth immigrants from Australia
had to compete {505} with these indigenous and well-established plants, and
only in a few cases were able to obtain a footing; whence it arises that we
have many Australian types, but few Australian species, in New Zealand, and
both phenomena are directly traceable to the combination of great powers of
dispersal with a high degree of adaptability. Exactly the same thing occurs
with the still more highly specialised Orchideae. These are not
proportionally so numerous in New Zealand (thirty-eight species), and this
is no doubt due to the fact that so many of them require
insect-fertilisation often by a particular family or genus (whereas almost
any insect will fertilise Compositae), and insects of all orders are
remarkably scarce in New Zealand.[136] This would at once prevent the
establishment of many of the orchids which may have reached the islands,
while those which did find suitable fertilisers and other favourable
conditions would soon become modif