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 goo 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 
