CH. Ill] BARRIERS TO DISPERSAL. 135 



A wide expanse of sea is an effectual barrier to the 

 mammalia and reptilia but not altogether to birds, 

 especially if there be islands which shorten the stretches 

 of ocean to be traversed. Even a comparatively small 

 tract of ocean, in some cases a narrow strait, opposes itself 

 as an insuperable barrier to some forms of life. The 

 Amphibia, for example, cannot suffer the contact of sea 

 water, which is also fatal to their eggs. Earthworms are 

 also killed by sea water. The narrowest strait is therefore 

 as efficient a check to the migrations of these animals as 

 is the widest ocean. High mountain ranges are also 

 hindrances sometimes quite effectual to the extension of 

 range of purely terrestrial animals ; partly perhaps on 

 account of temperature but probably more on account of 

 the physical obstacle. A great expanse of desert is often 

 as effective a barrier as a tract of sea. The desert of 

 Sahara separates two faunas that are widely different. 

 Changes in climate are also to some extent hindrances, 

 though not to so great a degree as other barriers. Many 

 animals can suffer with impunity an arctic or a tropical 

 climate; the tiger is often regarded as a purely tropical 

 creature, but as is well known it extends its range to 

 Amurland, in northern China. At the Zoological Society's 

 Gardens the polar bears do not show any great mortality ; 

 a specimen once lived there in perfefct health for 37 years. 

 Travellers have described monkeys leaping among the 

 snow-clad branches of pine trees upon the Himalayas. 

 Minute organisms have special facilities for passive mi- 

 gration. M. de Guerne 1 investigated the mud adherent to 

 1 Comptes Rendus, Soc. Biol. 1888. 



