150 SPECIAL FEATURES 



all. Islands which do not lie very near a continent, 

 and are separated from the nearest continent by 

 deep water, that is, do not lie upon the same con- 

 tinental shelf as the continent, show great general 

 similarity as regards their fauna. In such islands there 

 are no indigenous land mammals, often with the doubt- 

 ful exception of mice and rats, animals readily intro- 

 duced by man. Amphibians are also absent, a fact 

 readily explaiaed when we recollect that both the eggs 

 and adults are very intolerant of salt, and therefore 

 could not be readily transported across sea-water by 

 floating logs of wood or any similar means. In such 

 islands generally the commonest land forms are birds 

 and invertebrates, especially insects and land shells. 

 These in all cases tend to run into local races, djie 

 apparently to the isolation of the stock. Further, fly- 

 ing forms tend to lose their wings or to possess only 

 limited powers of flight. This is very marked in some 

 birds, thus the flightless dodo, the soHtaire, the kiwi of 

 New Zealand, and so forth are all inhabitants of islands. 

 It is noticeable that the power of flight tends specially 

 to diminish in animals found in islands much exposed 

 to hurricanes, for here even short flights might expose 

 the animals to strong winds, which would sweep them 

 out to sea, and thus destroy them, while those which 

 flew little or not at all would be more hkely to breed 

 and continue their own type. 



Reptiles are not very frequent in islands separated 

 by deep water from continents, and the presence of 

 the large tortoises in the Galapagos Islands is held by 

 some to indicate a connexion, at a remote period, with 

 the continent of America. Wallace, on the other hand, 

 believes that the ancestors of these forms were acci- 

 dentally introduced, possibly on floating timber. 



