324 OUTLINES OF EVOLUTIONARY BIOLOGY 



carried out to sea in storms, and although the great majority of 

 these will inevitably perish a few will occasionally manage to 

 reach some distant haven where they may succeed in establish- 

 ing a colony and thus extending the range of the species. 



During westerly winds American birds not infrequently make 

 their appearance on various parts of the coast of Europe, while 

 north of the 58th parallel of latitude the polar winds trend in 

 the opposite direction and with them we find a transference of 

 European birds, by way of Iceland and Greenland, to the 

 American continent. 1 During storms, again, European birds are 

 cast upon the Azores, abotit 1,000 miles from the nearest 

 continental coast, and there is strong reason for believing that 

 the little wax-eye (Zoster ops lateralls) has been transported in 

 this way from Australia to New Zealand, where it has succeeded 

 in establishing itself. 



Water currents may play an important part in the dispersal 

 of two groups of terrestrial animals those which occasionally 

 swim and those which are liable to be carried away on icebergs 

 or on floating masses of vegetation. Most quadrupeds swim well 

 and even if not habitual swimmers may be forced to take to the 

 water in times of flood. In this way they may cross large 

 rivers and even get carried out to sea and perhaps to some 

 neighbouring island, but they cannot cross large stretches 

 of open ocean, and are accordingly never found, except 

 when introduced by man, on islands far remote from any 

 continent. 



In polar regions the floating ice affords a means of dispersal 

 to such animals as wolves and polar bears, while within the 

 tropics floating islands or rafts formed of matted vegetation play 

 the same part. Such islands have been observed floating out to 

 sea from the mouths of large rivers like the Ganges, the Amazon, 

 the Congo and the Orinoco. They serve as a means of transport 

 to many different kinds of terrestrial reptiles, birds and mammals, 

 and countless molluscs, worms and insects, to say nothing of 

 plants. 



" If," says Sir Charles Lyell, " the surface of the deep be 

 calm, and the rafts are carried along by a current, or wafted by 

 some slight breath of air fanning the foliage of the green trees, 

 it may arrive, after a passage of several weeks, at the bay of an 

 island into which its plants and animals may be poured out as 



1 Heilprin, op. cit., p. 47. 



