GEOGRAPHICAL DISTRIBUTION OF ANIMALS 995 
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 
Rivers 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 River 
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. 
