562 



SCIENCE. 



[N. S. Vol. XIV. No. 354. 



This is true especially of the New England 

 region, the North Sea, the Gulf of Guinea 

 and the coast of Argentina. The fish fauna 

 of the North Atlantic is derived mainly 

 from the North Pacific, the differences 

 lying mainly in the lower richness of the 

 North Atlantic. But, in certain groups 

 common to the two regions, the migration 

 must have been in the opposite direction ; 

 exceptions that prove the rule. 



REALMS OF DISTRIBUTION OF FRESH- 

 WATER FISHES. 



If we consider the fresh-water fishes 

 alone we may divide the land areas of the 

 earth into districts and zones, fairly agree- 

 ing with those marked out for mammals 

 and birds. The river-basin, bounded by 

 its shores and the sea at its mouth, shows 

 many resemblances, from the point of view 

 of a fish, to an island considered as the 

 home of an animal. The nature of the 

 various barriers limiting species in river- 

 basins I have elsewhere * fully discussed 

 and need not consider it further here. It 

 is evident that, with fishes, the differences 

 in latitude outweigh those of continental 

 areas, and a primary division into Old 

 World and New World would not be ten- 

 able. 



The chief areas of dispersion of fresh- 

 water fishes we may indicate as follows, 

 following essentially the grouping proposed 

 by Dr. Giinther : f 



NORTHERN ZONE (ARCTIC AND TEMPERATE;. 



With Dr. Giinther, we may recognize, 

 first the Northern Zone, characterized famil- 

 iarly by the presence of sturgeon, salmon, 

 trout, white-fish, pike, lamprey, stickle- 

 back and other species of which the genera 

 and often the species are identical in 

 Europe, Siberia, Canada, Alaska and most 

 of the United States, Japan and China. 



" Science Sketches : ' The Dispersion of Fresh- 

 water Fishes.' 



t ' Introduction to the Study of Fishes. ' 



This is subject to cross- division into two 

 great districts, the first Europe- Asiatic, the 

 second North American. These two agree 

 very closely to the northward but diverge 

 widely to the southward, developing a 

 variety of specialized genera and species, 

 and both of them passing finally, by de- 

 grees, into the Equatorial Zone. 



Still another line of division is made by 

 the Ural Mountains in the Old World and 

 by the Rocky Mountains in the New. In 

 both cases the Eastern region is vastly 

 richer in genera and species, as well as in 

 autochthonous forms, than the Western. 

 The reason for this lies in the vastly greater 

 extent of the river basins of China and the 

 Eastern United States, as compared with 

 those of Europe or the Californian region. 



Minor divisions are those which separate 

 the Great Lake region from the streams 

 tributary to the Gulf of Mexico ; and in 

 Asia, those which separate China from 

 tributaries of the Caspian, the Black and 

 the Mediterranean. 



EQUATORIAL ZONE. 



The Equatorial Zone is roughly indicated 

 by the tropics of Cancer and Capricorn. Its 

 essential feature is that of the temperature, 

 and the peculiarities of its divisions are 

 caused by barriers of sea or mountains. 



Dr. Giinther finds the best line of separa- 

 tion into two divisions to lie in the presence 

 or absence of the great group of dace or 

 minnows,* to which nearly half of the 

 species of fresh-water fishes the world over 

 belong. The entire group, now spread every- 

 where except in the Arctic, South America, 

 Australia and the islands of the Pacific, had 

 its origin in India, from which its genera 

 have radiated in every direction. 



The Cyprinoid division of the Equatorial 

 Zone forms two districts, the Indian and 

 the African. The Acyprinoid division in- 

 cludes South America, south of Mexico, and 



* Cyprinidse. 



