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 be 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.^ 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 Compositge, notwithstanding their facilities for 

 dispersal by means of their feathered seeds ; but it has been 



^ It seems quite possible that the absence of pappus in this case is a recent 

 adaptation, and that it has 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. 



