368 
DARWINISM 
CHAP. 
already shown that limitations of area are almost always due 
to the competition of allied forms, facilities for dispersal being 
only one of many factors in determining the wide range of 
species. It is, however, a specially important factor in the 
case of the inhabitants of remote oceanic islands, since, whether 
they are peculiar species or not, they or their remote ancestors 
must at some time or other have reached their present posi¬ 
tion by natural means. 
I have already shown elsewhere, that the flora of the 
Azores strikingly supports the view of the species having been 
introduced by aerial transmission only, that is, by the agency 
of birds and the wind, because all plants that could not possibly 
have been carried by these means are absent. 1 In the same 
way we may account for the extreme rarity of Leguminosse in 
all oceanic islands. Mr. Hemsley, in his Report on Insular 
Floras, says that they “are wanting in a large number of 
oceanic islands where there is no true littoral flora,” as St. 
Helena, Juan Fernandez, and all the islands of the South 
Atlantic and South Indian Oceans. Even in the tropical 
islands, such as Mauritius and Bourbon, there are no endemic 
species, and very few in .the Galapagos and the remoter Pacific 
Islands. All these facts are quite in accordance with the absence 
of facilities for transmission through the air, either by birds 
or the wind, owing to the comparatively large size and weight 
of the seeds ; and an additional proof is thus afforded of the 
extreme rarity of the successful floating of seeds for great 
distances across the ocean. 2 
Explanation of North Temperate Plants in the Southern Hemisphere. 
If w r e now admit that many seeds which are either minute 
in size, of thin texture or Avavy form, or so fringed or 
margined as to afford a good hold to the air, are capable of 
being carried for many hundreds of miles by exceptionally 
1 See Island Life, p. 251. 
- Mr. Hemsley suggests that it is not so much the difficulty of transmission 
by floating, as the bad conditions the seeds are usually exposed to when they 
reach land. Many, even if they germinate, are destroyed by the waves, as 
Burchell noticed at St. Helena ; while even a flat and sheltered shore would 
be an unsuitable position for many inland plants. Air-borne seeds, on the 
other hand, may be carried far inland, and so scattered that some of them 
are likely to reach suitable stations. 
