xii GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 373 



times greater by the latter mode than by the former. 1 We 

 have seen that inorganic particles of much greater specific 

 gravity than seeds, and nearly as heavy as the smallest kinds, 

 are carried to great distances through the air, and we can 

 therefore hardly doubt that some seeds are carried as far. 

 The direct agency of the wind, as a supplement to bird- 

 transport, will help to explain the presence in oceanic islands 

 of plants growing in dry or rocky places whose small seeds 

 are not likely to become attached to birds ; while it seems to 

 be the only effective agency possible in the dispersal of those 

 species of alpine or sub-alpine plants found on the summits 

 of distant mountains, or still more widely separated in the 

 temperate zones of the northern and southern hemispheres. 



Concluding Remarks. 



On the general principles that have been now laid down, it 

 will be found that all the chief facts of the geographical dis- 

 tribution of animals and plants can be sufficiently understood. 

 There will, of course, be many cases of difficulty and some 

 seeming anomalies, but these can usually be seen to depend on 

 our ignorance of some of the essential factors of the problem. 

 Either we do not know the distribution of the group in recent 

 geological times, or we are still ignorant of the special methods 

 by which the organisms are able to cross the sea. The latter 

 difficulty applies especially to the lizard tribe, which are found 



1 A very remarkable case of wind conveyance of seeds on a large scale is 

 described in a letter from Mr. Thomas Hanbury to bis brother, the late 

 Daniel Hanbnry, which has been kindly communicated to me by Mr. Hemsley 

 of Kew. The letter is dated "Shanghai, 1st May 1856," and the passage 

 referred to is as follows : — 



" For the past three days we have had very warm weather for this time of 

 year, in fact almost as warm as the middle of summer. Last evening the 

 wind suddenly changed round to the north and blew all night with consider- 

 able violence, making a great change in the atmosphere. 



"This morning, myriads of small white particles are floating about in the 

 air ; there is not a siugle cloud and no mist, yet the sun is quite obscured by 

 this substance, and it looks like a white fog in England. I enclose thee a 

 sample, thinking it may interest. It is evidently a vegetable production ; I 

 think, apparently, some kind of seed." 



Mr. Hemsley adds, that this substance proves to be the plumose seeds of 

 a poplar or willow. In order to produce the effects described — quite obscuring 

 the sun like a white fog, — the seeds must have filled the air to a very great 

 height ; and they must have been brought from some district where there were 

 extensive tracts covered with the tree which produced them. 



