GEOGRAPHIC ISOLATION 95 



Fresh water is an impassable barrier for many marine animals 

 and plants. Animals which were able to adapt themselves to fresh 

 water have thus been able to survive in it long after they became 

 extinct in the sea. The simple coelenterates, Hydra and Microhydra 

 and their relatives, are probably preserved only because of their 

 adaptation to the fresh-water environment. The ganoids which domi- 

 nated the seas in the Mesozoic survive primarily in fresh waters. The 

 three remaining genera of the extremely ancient lungfishes are fresh- 

 water forms. The more primitive bony fishes, the physostomes, make 

 up a large majority of fresh-water fishes, where only a few of the 

 spiny-rayed Acanthopterygii have followed them. This array of 

 archaic fishes in the fresh waters has led some to play with the 

 idea that bony fishes originated here and later migrated to the sea. 



When such a protecting barrier is broken down, as happens 

 nowadays chiefly through the agency of man, the primitive types are 

 usually doomed. When, by accident or intent, modern animals are 

 introduced in the isolated areas, they promptly demonstrate their 

 competitive superiority over primitive inhabitants. Thus the marsupials 

 of Australia are disappearing before the introduced cattle, sheep, 

 rabbits, housecats, and foxes of the settlers. The endemic birds of 

 New Zealand give way before the buntings, starlings, and goldfinches. 

 In all parts of the southern hemisphere the native earthworms dis- 

 appear with the introduction of Lumbricidae. 



When man himself becomes an enemy, the effects are still more 

 severe. The list of animals exterminated by his thoughtless action 

 is long. Steller's sea cow (Rhytina) , the great auk, the dodo, and 

 solitaire on Mauritius and Bourbon, the passenger pigeon, the quagga, 

 and many more are gone, and still others are in danger of extinction. 

 Even the composition of the marine fauna is altered by the influence 

 of man. Whales and seals disappear in consequence of hunting, and 

 lobsters and soles are diminished in size and in numbers on the fish- 

 ing grounds. The same phenomenon has taken place with primitive 

 human races, many of which have been exterminated, displaced, or 

 absorbed by European or other dominant peoples. The original faunal 

 conditions are to be found only in distant wildernesses, in high moun- 

 tains, in tropical forests, and in the depths of the sea. 



BIBLIOGRAPHY 



1) Wagner, 1889, Entstehung der Art en .—2) Romanes, 1897. Darwin ami 

 after Darwin.— 3) Iwanow & Philiptschenko, 1916, Z. indnkt. Abst.-Lehre, 16, 

 p. 5 ff _ 4) Dunn, 1926, Proc. Boston Soc. Nat. Hist., 38, p. 111-130.— 5) Jordan, 

 1906. Bull. Bur. Fisheries, 25, p. 177.— 6) Ortmann, 1906. Science. 23. p. 504 ff.— 



