EXTENSION OF EVOLUTIONARY THEORIES 329 



species can be so altered by isolation as to be incapable 

 of crossing with the parental species. In fact, long 

 before man crossed the waves on his vessels, organisms 

 were conveyed into remote districts. Sometimes small 

 water-snails clung to the legs and feathers of swimming 

 birds, and were taken far across the sea to strange 

 islands, where they entered the water once more and 

 found their proper conditions. In the same way other 

 aquatic animals travelled far and wide — the minute 

 Gggs, especially, that were easily caught in the feathery 

 coats of the swimming birds and taken away uncon- 

 sciously. Land animals also could be dispersed by 

 birds in this way. 



It was by no means necessary to have living vehicles 

 to convey animals into distant and isolated districts. It 

 might often happen that a branch of a tree with small 

 animals sitting on it would fall into the river, be carried 

 out to sea, and be borne by the currents to distant 

 localities. In this way, as a matter of fact, branches 

 have come from America to the coast of Europe. The 

 eggs, especially, which are comparatively insensitive to 

 external influences, and the seeds of plants, might be 

 conveyed in this way. It is certain that many newly- 

 formed islands have been thus populated. 



Not only may the sea transport germs in this way, 

 but many a flying animal has been taken a considerable 

 way by stormy winds. Insects and birds often get into 

 currents of air that carry them to distant localities, and 

 they often find such a favourable environment there that 

 they do not think of returning even when it is possible 

 to do so. 



