ISLAND COMMUNITIES 519 



forms constitute the main part of insular fish faunae. Some of these 

 are able to enter fresh water directly ; others are represented by closely 

 allied forms in the fresh-water habitat,* The species of Galaxias, wide- 

 spread in the southern hemisphere, belong in this category, since they 

 have marine relatives, and one species at least, G. attenuatus of New 

 Zealand, descends to the sea to breed. Few areas have so many rivers 

 as San Thome in the Gulf of Guinea, but this island has only 2 species 

 of fresh-water fishes, both gobies and hence undoubtedly derived from 

 the sea. 7 Even the blind cave fishes of Cuba belong to the marine 

 family Brotulidae. 8 In accordance with its habit of spawning in the 

 sea, the common eel and its allies are widely present on islands. It is 

 the only fresh-water fish in the Azores, Madeiras, and in Sardinia. 



The effects of isolation appear furthermore in the independent fur- 

 ther development of the insular populations separated from the parent 

 stock. Mutations in the island stock may become fixed while in the 

 parent stock they may well be swamped by crossing. Islands accord- 

 ingly will have the more endemic forms the more effectively and the 

 longer they have been separated from the nearest inhabited area. The 

 length of time through which the isolation has continued is the prin- 

 cipal factor, and it makes little difference whether the island is of 

 oceanic or continental origin. Ancient continental as well as ancient 

 oceanic islands are contrasted with youthful islands by their great 

 number of endemic species, genera, and even families. In the British 

 Isles a divergence of the mammals and birds from those of central 

 Europe is discoverable only by detailed study. In Madagascar, on the 

 other hand, all the non-flying members of these classes, except the 

 species of Potamochoerus, Hippopotamus, Crocidura, and Fossa, be- 

 long to genera or even families unknown elsewhere. 9 The fauna of 

 Krakatoa, which has been entirely acquired since 1887, does not have 

 a single endemic form, while on the Canary Islands 133 out of 167 

 land mollusks are endemic, with a somewhat smaller proportion of 

 endemism in insects, f 



The proportion of endemic species to the total increases with dis- 

 tance from the mainland, with the age of the islands, and probably 

 with other factors. The Azores, 1400 km. from Europe, have 69 species 

 of land snails, of which 32 are endemic, 212 beetlts with 14 endemic, 

 and 30 land birds, of which only one has developed into a species con- 

 fined to the islands. 10 By contrast, the Hawaiian Islands, 3000 km. 



* Gobiidao, Percidae, Mugilidae, and Poecilidae afford examples. 



t The distribution of land mollusks is thought by many students to require 

 land bridges and to indicate that even the oceanic islands of the Pacific are 

 vestiges of a former continent (see Chapter VII). 



