THE AQUATIC VERTEBRATES 1053 



are not as varied as those adjusted to the warmer, more accessible 

 waters of the central and southern lowlands. The latter are the 

 homes of the black bass, sunfishes, catfishes, gar pikes, and others. 



So called warm-water species are capable of a wide range of 

 adjustment to differing climates. Wherever a north and south 

 river connects warmer with colder areas in which the species are 

 otherwise different, the easy route of migration induces some species 

 to extend their range into otherwise shunned areas. The Missis- 

 sippi has induced a southward migration of several species beyond 

 their normal range, the Nile has extended the equatorial African 

 fauna to its mouth. But the most notable example is offered by 

 the Madeira and Tapajos to Paraguay and La Plata waterway. 

 It extends from the equator south to a latitude equal to that of 

 Memphis. Nearly all of the fishes of Buenos Aires belong to Ama- 

 zonian genera or even species. Only one or two Amazonian genera 

 have succeeded in reaching the borders of the United States. 

 Fifty Amazonian genera have reached the La Plata basin that have 

 not succeeded in going an equal distance south on the Atlantic 

 coast where they did not have the faciUties or inducement of a 

 continuous waterway. 



Current and Adjustment to It. — The major adaptation of all 

 fresh-water fishes is to the locomotion of water. Most fishes 

 stand head upstream, a position that makes the respiratory move- 

 ment easiest in a current. Different fishes, and in some cases the 

 same fishes at different seasons of the year, are adjusted to the 

 entire range of variation in the intensity of the locomotion. Water- 

 falls only are not inhabited by fishes, but even these are ascended 

 or descended if not too high. Different species of the Salmonidae 

 give us examples of the entire range of adjustment to currents. 

 Some of them hve only in the stagnant water of deep lakes. Others 

 live only in swift mountain brooks. The members of the genus 

 Coregonus usually live in the stagnant waters of lakes, but Core- 

 gonus couUeri is found in a mountain torrent, and the Tippecanoe 

 Cisco, which hves in the stagnant water of the lake during the 

 greater part of the year, runs up the tributaries in December. 

 Other whitefishes and trout have the same habit. Different fishes 

 are even adjusted to the differences in the same smaU stream in 



