. Murray, and others five times, eight times, and fifteen times as
heavy; but they are proportionately very much larger, and, being usually
irregular in shape or compressed, they expose a very much larger surface
to the air. The surface is often rough, and several have dilated margins
or tailed appendages, increasing friction and rendering the uniform rate
of falling through still air immensely less than in the case of the
smooth, rounded, solid quartz grains. With these advantages it is a
moderate estimate that seeds ten times the weight of the quartz grains
could be carried quite as far through the air by a violent gale and
under the most favourable conditions. These limits will include five of
the seeds here given, as well as hundreds of others which do not exceed
them in weight; and to these we may add some larger seeds which have
other favourable characteristics, as is the case with numbers 11-13,
which, though very much larger than the rest, are so formed as in all
probability to be still more easily carried great distances by a gale of
wind. It appears, therefore, to be absolutely certain that every
autumnal gale capable of conveying solid mineral particles to great
distances, must also carry numbers of small seeds at least as far; and
if this is so, the wind alone will form one of the most effective agents
in the dispersal of plants.

Hitherto this mode of conveyance, as applying to the transmission of
seeds for great distances across the ocean, has been rejected by
botanists, for two reasons. In the first place, there is said to be no
direct evidence of such conveyance; and, secondly, the peculiar plants
of remote oceanic islands do not appear to have seeds specially adapted
for aerial transmission. I will consider briefly each of these
objections.

_Objection to the Theory of Wind-Dispersal._

To obtain direct evidence of the transmission of such minute and
perishable objects, which do not exist in great quantities, and are
probably carried to the greatest distances but rarely and as single
specimens, is extremely difficult. A bird or insect can be seen if it
comes on board ship, but who would ever detect the seeds of Mimulus or
Orchis even if a score of them fell on a ship's deck? Yet if but one
such seed per century were carried to an oceanic island, that island
might become rapidly overrun by the plant, if the conditions were
favourable to its growth and reproduction. It is further objected that
search