XII 
GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 367 
of the wind observed during twelve hours at the Ben Nevis 
observatory, while the velocity sometimes rises to 120 miles 
an hour. A twelve hours’ gale might, therefore, carry 
light seeds a thousand miles as easily and certainly as it 
could carry quartz-grains of much greater specific gravity, 
rotundity, and smoothness, 500 or even 100 miles; and it is 
difficult even to imagine a sufficient reason why they should 
not he 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 
orchidese, 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. 1 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 Composite, notwithstanding their facilities for 
dispersal by means of their feathered seeds; but it has been 
1 It seems quite possible that the absence of pappus in this case is a recent 
adaptation, and that it lias been brought about by causes similar to those 
which have reduced or aborted the wings of insects in oceanic islands. For 
when a plant has once reached one of the storm-swept islands of the southern 
ocean, the pappus will be injurious for the same reason that the wings of 
insects are injurious, since it will lead to the seeds being blown out to sea and 
destroyed. The seeds which are heaviest and have least pappus will have the 
best chance of falling on the ground and remaining there to germinate, and 
this process of selection might rapidly lead to the entire disappearance of the 
pappus. 
