smoothness, 500 or even 100 miles; and
it is difficult even to imagine a sufficient reason why they should not
be so carried--perhaps very rarely and under exceptionally favourable
conditions,--but this is all that is required.

As regards the second objection, it has been observed that orchideae,
which have often exceedingly small and light seeds, are remarkably
absent from oceanic islands. This, however, may be very largely due to
their extreme specialisation and dependence on insect agency for their
fertilisation; while the fact that they do occur in such very remote
islands as the Azores, Tahiti, and the Sandwich Islands, proves that
they must have once reached these localities either by the agency of
birds or by transmission through the air; and the facts I have given
above render the latter mode at least as probable as the former. Sir
Joseph Hooker remarks on the composite plant of Kerguelen Island (Cotula
plumosa) being found also on Lord Auckland and MacQuarrie Islands, and
yet having no pappus, while other species of the genus possess it. This
is certainly remarkable, and proves that the plant must have, or once
have had, some other means of dispersal across wide oceans.[177] One of
the most widely dispersed species in the whole world (Sonchus oleraceus)
possesses pappus, as do four out of five of the species which are common
to Europe and New Zealand, all of which have a very wide distribution.
The same author remarks on the limited area occupied by most species of
Compositae, notwithstanding their facilities for dispersal by means of
their feathered seeds; but it has been already shown that limitations
of area are almost always due to the competition of allied forms,
facilities for dispersal being only one of many factors in determining
the wide range of species. It is, however, a specially important factor
in the case of the inhabitants of remote oceanic islands, since, whether
they are peculiar species or not, they or their remote ancestors must at
some time or other have reached their present position by natural means.

I have already shown elsewhere, that the flora of the Azores strikingly
supports the view of the species having been introduced by aerial
transmission only, that is, by the agency of birds and the wind, because
all plants that could not possibly have been carried by these means are
absent.[178] In the same way we may account for the extreme rarity of
Leguminosae in all oceanic islands. Mr.