s. For each
single seed carried away by external attachment to the feet or feathers
of a bird, countless millions are probably carried away by violent
winds; and the chance of conveyance to a great distance and in a
definite direction must be many times greater by the latter mode than
by the former.[182] We have seen that inorganic particles of much
greater specific gravity than seeds, and nearly as heavy as the smallest
kinds, are carried to great distances through the air, and we can
therefore hardly doubt that some seeds are carried as far. The direct
agency of the wind, as a supplement to bird-transport, will help to
explain the presence in oceanic islands of plants growing in dry or
rocky places whose small seeds are not likely to become attached to
birds; while it seems to be the only effective agency possible in the
dispersal of those species of alpine or sub-alpine plants found on the
summits of distant mountains, or still more widely separated in the
temperate zones of the northern and southern hemispheres.

_Concluding Remarks._

On the general principles that have been now laid down, it will be found
that all the chief facts of the geographical distribution of animals and
plants can be sufficiently understood. There will, of course, be many
cases of difficulty and some seeming anomalies, but these can usually be
seen to depend on our ignorance of some of the essential factors of the
problem. Either we do not know the distribution of the group in recent
geological times, or we are still ignorant of the special methods by
which the organisms are able to cross the sea. The latter difficulty
applies especially to the lizard tribe, which are found in almost all
the tropical oceanic islands; but the particular mode in which they are
able to traverse a wide expanse of ocean, which is a perfect barrier to
batrachia and almost so to snakes, has not yet been discovered. Lizards
are found in all the larger Pacific Islands as far as Tahiti, while
snakes do not extend beyond the Fiji Islands; and the latter are also
absent from Mauritius and Bourbon, where lizards of seven or eight
species abound. Naturalists resident in the Pacific Islands would make a
valuable contribution to our science by studying the life-history of the
native lizards, and endeavouring to ascertain the special facilities
they possess for crossing over wide spaces of ocean.

FOOTNOTES:

[Footnote 163: See A. Agassiz, _Three Cruises of the Blake_