GEOGRAPHICAL DISTRIBUTION OF ANIMALS 295 



waterfalls have checked the movements of many species, 

 while others have been helped by artificial channels or 

 canals. Streams that run muddy at times are not favor- 

 able for animal life. Still less favorable is the condition 

 frequent in the arid region in which streams are full to 

 the banks in the rainy season and shrunk to detached 

 pools in the dry months. 



The stream that has the greatest variety of animals in it 

 would be one (1) connected with a large river, (2) in a warm 

 climate, (3) with clear water and (4) little fluctuation from 

 winter to summer, (5) with little change in the clearness of 

 the water, (6) a gravelly bottom, (7) preferably of lime- 

 stone, and (8) covered in its quiet reaches and its ripples 

 with water-weeds. These conditions are best realized in 

 tributaries of the Ohio, Cumberland, Tennessee, and Ozark 

 Eivers among American streams, and it is in them that the 

 greatest number of species of fresh-water animals (fishes, 

 cray-fishes, mussels, etc.) has been recorded. These streams 

 approach most nearly to the ideal homes for animals of the 

 fresh waters. The streams of Wisconsin, Michigan, and the 

 Columbia region have many advantages, but are too cold. 

 Those of Illinois, Iowa, northern Missouri, and Kansas are 

 too sluggish, and sometimes run muddy. Those of Texas 

 and California shrink too much in summer, and are too 

 isolated The streams of the Atlantic coast are less iso- 

 lated, but none connect with a great basin, and those of 

 New England run too cold for the great mass of the spe- 

 cies. For similar reasons the fresh-water animal life of 

 Europe is relatively scanty, that of the Danube and Volga 

 being richest. The animal life of the fresh water of South 

 America centers in the Amazon, and that of Africa in the 

 Nile, the Niger, and the Congo. The great rivers of Si- 

 beria, like the Yukon in Alaska and the Mackenzie Eiver 

 in British America, have but few forms of fresh- water ani- 

 mals, though those kinds fitted for life in cold, clear water 

 exist in great abundance. 



