passed these barriers. The falls of the Cumberland limit the 

 range of most of the larger fishes of the river, but the streams 

 above it have their quota of darters and minnows. It is evi- 

 dent that the past history of the stream must enter as a factor 

 into this discussion, but this past history it is not always possi- 

 ble to trace. Dams or artificial waterfalls now check the free 

 movement of many species, especially those of migratory habits ; 

 while, conversely, numerous other species have extended their 

 range through the agency of canals (thus, Dorosoma cepedianum 

 Le Sueur, and Clupea chrysochloris Rafinesque, have found 

 their way into Lake Michigan through canals). 



Every year fishes are swept down the rivers by the winter's 

 floods ; and in the spring, as the spawning season approaches, 

 almost every species is found working its way up the stream. 

 In some cases, notably the Quinnat salmon {Oncorhynchtis 

 tschawytscha Walbaum) and the blueback salmon [Oncorhyn- 

 chus nerka Walbaum), the length of these migrations is 

 surprisingly great. To some species rapids and shallows have 

 proved a sufficient barrier, and other kinds have been kept back 

 by unfavorable conditions of various sorts. Streams whose 

 waters are always charged with silt or sediment, as the Missouri, 

 Arkansas, or Brazos, do not invite fishes ; and even the occa- 

 sional floods of red mud such as disfigure otherwise clear 

 streams, like the Red river or the Colorado (of Texas), are 

 unfavorable. Extremely unfavorable also is the condition 

 which obtains in many rivers of the Southwest ; as for example, 

 the Red river, the Sabine, and the Trinity, which are full from 

 bank to bank in winter and spring, and which dwindle to mere 

 rivulets in the autumn droughts. 



In general, those streams which have conditions most favor- 

 able to fish-life will be found to contain the greatest number of 

 species. Such streams invite immigration ; and in them the 

 struggle for existence is individual against individual, species 

 against species, and not a mere struggle with hard conditions of 

 life. Some of the conditions most favorable to the existence in 

 any stream of a large number of species of fishes are the follow- 

 ing, the most important of which is the one mentioned first : 

 connection with a large hydrographic basin ; a warm climate ; 



