Geographical Distribution. 217 



terrestrial organisms ; and, in the case of aquatic 

 organisms, the presence of land. But it is to be 

 observed that, as regards marine organisms, any con- 

 siderable difference in the temperature of the water 

 may constitute a barrier as effectual as the presence 

 of land ; and also that, in the case of all shallow- 

 water faunas, a tract of deep ocean constitutes almost 

 as complete a barrier as it does to terrestrial faunas. 



Now, the means whereby barriers admit of being 

 accidentally or occasionally surmounted are, of course, 

 various ; and they differ in the case of different 

 organisms. Birds, bats, and insects, on account of 

 their powers of flight, are particularly apt to be blown 

 out great distances to sea, and hence of all animals 

 are most likely to become the involuntary colonists of 

 distant shores. Floating timber serves to convey 

 seeds and eggs of small animals over great distances ; 

 and Darwin has shown that many kinds of seeds are 

 able of themselves to float for more than a month in 

 sea-water without losing their powers of germination. 

 For instance, out of 87 kinds, 64 germinated after an 

 immersion of 28 days, and a few survived an immersion 

 of 137 days. As a result of all his experiments he 

 concludes, that the seeds of at least ten per cent, of the 

 species of plants of any country might be floated by 

 sea-currents during 28 days, without losing their 

 powers of germination ; and this, at the average rate 

 of flow of several Atlantic currents, would serve to 

 transport the seeds to a distance of at least 900 miles. 

 Again, he proved that even seeds which are quickly 

 destroyed by contact with sea-water admit of being 

 successfully transported during 30 days, if they be 

 contained within the crop of a dead bird. He also 



