summits, or for that "continuous current of
vegetation" described by Sir Joseph Hooker as having apparently long
existed from the northern to the southern hemisphere. It may be admitted
that we can, possibly, account for the greater portion of the floras of
remote oceanic islands by the agency of birds alone; because, when blown
out to sea land-birds must reach some island or perish, and all which
come within sight of an island will struggle to reach it as their only
refuge. But, with mountain summits the case is altogether different,
because, being surrounded by land instead of by sea, no bird would need
to fly, or to be carried by the wind, for several hundred miles at a
stretch to another mountain summit, but would find a refuge in the
surrounding uplands, ridges, valleys, or plains. As a rule the birds
that frequent lofty mountain tops are peculiar species, allied to those
of the surrounding district; and there is no indication whatever of the
passage of birds from one remote mountain to another in any way
comparable with the flights of birds which are known to reach the Azores
annually, or even with the few regular migrants from Australia to New
Zealand. It is almost impossible to conceive that the seeds of the
Himalayan primula should have been thus carried to Java; but, by means
of gales of wind, and intermediate stations from fifty to a few hundred
miles apart, where the seeds might vegetate for a year or two and
produce fresh seed to be again carried on in the same manner, the
transmission might, after many failures, be at last effected.

A very important consideration is the vastly larger scale on which
wind-carriage of seeds must act, as compared with bird-carriage. It can
only be a few birds which carry seeds attached to their feathers or
feet. A very small proportion of these would carry the seeds of Alpine
plants; while an almost infinitesimal fraction of these latter would
convey the few seeds attached to them safely to an oceanic island or
remote mountain. But winds, in the form of whirlwinds or tornadoes,
gales or hurricanes, are perpetually at work over large areas of land
and sea. Insects and light particles of matter are often carried up to
the tops of high mountains; and, from the very nature and origin of
winds, they usually consist of ascending or descending currents, the
former capable of suspending such small and light objects as are many
seeds long enough for them to be carried enormous distance