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 his brother, the late 
Daniel Hanbury, 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. 
' 1 This morning, myriads of small white particles are floating about in the 
air ; there is not a single 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. 
