Islands 45 



the sea has been invaded from fresh water or from land by bony 

 fishes, pinnipeds, several orders of reptiles, and other animals. 



Under experimental conditions fishes have been gradually ad- 

 justed to water of higher salinity than the ocean. Richet (1926) 

 kept a fish, Diplodus sargus (Gmelin) , for 7.5 months as the salt 

 content of the medium was raised to 52.1 grams per liter. The fish 

 died when the salinity was decreased to 31.0. In Japan a student 

 working in Professor N. Yatsu's laboratory in 1930 showed the 

 writer specimens of the fish, Oryzias latipes (Temminck 6C Schle- 

 gel) , which were living in small aquaria in which salinities had been 

 gradually increased. Some had reached salinities as high as 60.0. 

 Rees (1941) in rock pools on the shores of Wales observed Mono- 

 celis jus^a. When transferred from normal sea water to higher 

 salinities the worm endured those as high as 76.d>, but when water 

 was allowed to evaporate gradually it lived in 109-120. Certain 

 insects are able to live in high salinities. Ephydrid fly larvae live in 

 the Great Salt Lake, Utah. In Japan the mosquito larvae live in 

 littoral rock pools in salinities as high as 42.0 (Pearse, 1931). At 

 Dry Tortugas mosquito and dragon-fly larvae live in salinities as 

 high as 72.0 and 62.0, respectively (Pearse, 1932e) . Animals which 

 live in solutions which contain salts or in solutions which have 

 osmotic pressures which are above or below those of their own 

 body fluids do not vary internally as the surrounding medium varies 

 so that they become isotonic with it. They are animals that have 

 attained some degree of internal stability and thus become more or 

 less independent of the environment (Pike & Scott, 1915; Pearse, 

 1931, 1932b). There have been many ad- and ab-oceanic migra- 

 tions in the past and such are taking place today. Animals along 

 beaches in general are able to endure greater variations in environ- 

 ment than those in estuaries and hence more often become estab- 

 lished in media which differ from those in which their ancestors 

 lived. 



