368 DARWINISM 



already shown that limitations of ai-ea arc 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 inhaljitants 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 sjiecies 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.^ 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 wantijig in a large number of 

 oceanic islands Avhere 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. ^ 



Explcvnation of Nmih Temperate Plants in the Southern Hemisphere. 



If we now admit that many seeds which are either minute 

 in size, of thin texture or wavy 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 



^ 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. 



