DISTKIBUTION OF MAMMALS 163 



ESTABLISHMENT OF SPECIES ON ISLANDS 



Few of those surviving species are able to establish them- 

 selves permanently. The reason for this is that the new environ- 

 ment must not only be favorable and provide approximately 

 the kind of climate and the kind of food the animal needs, but 

 the newcomer must either arrive at the same time as one of its 

 opposite sex or be a pregnant female in order that the process 

 of colonization of the island may begin. It is not surprising 

 then that the species of animals on all islands except those 

 near mainlands are few and assorted. A few mammals have 

 traveled as accidental "hitch-hikers" on native canoes and on the 

 ships of explorers, traders, and whalers. Several species of 

 rats have by this means spread to almost all parts of the world, 

 including even the most isolated islands. Domestic animals that 

 escape and go wild also add island species. The game animals 

 of various lands have been introduced, especially into New Zea- 

 land where they flourish. 



More species are found on large islands than small ones. 

 This is not merely because of the larger size of those islands but 

 especially because, offering a far wider choice of habitat, they 

 are likely to meet the requirements of specialized kinds of mam- 

 mals. Almost continuous invasion by random drift may be 

 imagined on islands which are favorably situated. However, 

 very small populations anywhere are liable to extermination, 

 and island populations are usually small. Large mammals are 

 especially liable to this risk. On a small island their numbers 

 are so limited that disease, an unfavorable year, or hunting by 

 human inhabitants may kill them all. The fauna of an island 

 may thus have formerly included species that were unable to 

 persist. 



ARCHAIC MAMMALS PRESERVED ON ISLANDS 



Some of the ancient types of animals found upon deep-sea 

 islands are the altered descendants of colonizers which must 



