CHAP. XII.] 



THE AZOEES. 



249 



carried enormous distances. An immense number are specially 

 adapted to be carried by the wind, through the possession of 

 down or hairs, or membranous wings or processes ; while others 

 are so minute, and produced in such profusion, that it is difficult 

 to place a limit to the distance they might be carried by gales 

 of wind or hurricanes. Another class of somey^hat heavier 

 seeds or dry fruits are capable of being exposed for a long 

 time to sea- water without injury. Mr. Darwin made many 

 experiments on this point, and he found that many seeds, 

 especially of Atriplex, Beta, oats, Capsicum, and the potato, 

 grew after 100 days' immersion, while a large number survived 

 fifty days. But he also found that most of them sink after a few 

 days' immersion, and this would certainly prevent them being 

 floated to very great distances. It is very possible, however, 

 that dried branches or flower-heads containing seeds would float 

 longer, while it is quite certain that many tropical seeds do float 

 for enormous distances, as witness the double cocoa-nuts which 

 cross the Indian ocean from the Seychelle Islands to the coast 

 of Sumatra, and the West Indian beans which frequently reach 

 the west coast of Scotland. There is therefore ample evidence 

 of the possibility of seeds being conveyed across the sea for 

 great distances by winds and surface currents. ■'• 



1 Some of Mr, Darwin's experiments are very interesting and suggestive. 

 Eipe hazel-nuts sank immediately, but when dried they floated for ninety 

 days, and afterwards germinated. An asparagus-plant with ripe berries, 

 when dried, floated for eiglity-nve days, and the seeds afterwards germi- 

 nated. Out of ninety-four dried plants experimented with, eighteen floated 

 for more than a month, and some for three months, and their powers of 

 germination seem never to have been wholly destroyed. Now, as oceanic 

 currents vary from thirty to sixty miles a day, such plants under the most 

 favourable conditions might be carried 90 x 60 ~ 5,400 miles 1 But even half 

 of this is ample to enable them to reach any oceanic island, and we must re- 

 member that till completely water-logged they might be driven along at a 

 much greater rate by the wind. Mr. Darwin calculates the distance by the 

 average time of flotation to be 924 miles ; but in such a case as this we 

 are entitled to take the extreme cases, because such countless thousands of 

 plants and seeds must be carried out to sea annually that the extreme cases 

 in a single experiment with only ninety-four plants^ must happen hundreds 

 or thousands of times and with hundreds or thousands of species, naturally, 

 and thus afford ample opportunities for successful migration. (See Origin 

 of Speciesy Gth Edition, p. 325.) 



