490 SOCIAL LIFE OF ANIMALS 



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Barriers. — (i) The salt of the sea proves a barrier to fresh water 

 animals. Amphibia are almost never found in salt water. It has 

 been found that the salinity of water determines the species of salt 

 marsh mosquitoes in a given region, while the Ph of the water is 

 apparently important in determining the distribution of Arthropod 

 species and of fishes. Sometimes species changes occur as in the 

 brine shrimp, Artemia, which alters its characteristics in fresh water 



(p. 170)- 



(2) Elton quotes various authors who have shown that water 



supply is an important limiting factor in determining the ranges of 

 wild birds and mammals. In the Burmese forests the occurrence of 

 elephants, bufFalos, tigers, panthers, pigs and monkeys is deter- 

 mined in dry season by the proximity of water-holes. In the case 

 of rooting and digging animals, such as pigs and moles, distribution 

 is determined by softness of the ground which of course depends on 

 the water supply. In Mesopotamia the black partridge {Franco- 

 linus vulgaris) is never found more than a hundred yards from water, 

 which it requires for drinking purposes. 



(3) Depth of water proves an effective barrier. Certain of the 

 shore fishes of Hawaii are not found in the waters of California. 



(4) Temperature proves most powerful in preventing the dis- 

 persal of many animals and plants. Some animals are found in 

 hot springs or in the Arctic ice, however. 



Dr. Nellie Payne has given us important information on the 

 survival of insects at low temperatures. Consult Payne, N. M,, 

 1926 (Freezing and Survival of Insects at Low Temperatures, 

 Quart. Rev. of Biol., vol. i, pp. 270-282). Brues, C. T. (Animal 

 Life in Hot Springs, Qu. Rev. of Biol., vol. 2, no. 2, pp. 181-203, 

 June, 1927), has summarized the literature and shown that Ameha 

 Umax was found in water at 50° to 52° C. (ii2°-i26° F.), while 

 Ciliates were found in water at 46° C. (115° F.). The crustacean 

 Cyclops was found in water at 36° C. and turtles appeared in water 

 at 44° C. (i 1 1° F.). Gudger, E. W. (Snow Worms, Nat. Hist., vol. 

 23, no. 5, pp. 450-456, 1923), cites cases of Enchytraeid worms and 

 other annelids besides insect larvae occurring in snow and ice. 



(5) Mountain ranges, with the factors of temperature and amount 

 of oxygen available, prove most effective barriers to many forms. 

 Thus, in some cases, individuals have been unable to reach new 

 regions because of barriers; in other instances they reach the new 

 field but are unable to survive because of lack of adaptation and 



