XIX 
GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 
365 
find that several are from twice to three times the weight 
of the grains found by Mr. Murray, and others five times, 
eight times, and fifteen times as heavy; but they are pro¬ 
portionately 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 
