FRESH-WATER FISHES— MYERS 343 



water, which binds them to the land as securely as any known animals 

 Secondly, on the land, they are inescapably confined to their own 

 particular drainage systems and can migrate from one isolated stream 

 basin to the next only through the slow physiographical change of 

 the land itself (stream capture, etc.). Throughout the world the 

 migrations of fresh-water fishes over extensive continental areas have 

 generally been excessively slower than those of almost any creature 

 that can creep, crawl, walk, or fly, however closely that creature 

 may have been bound by its ecological tolerances. This is exception- 

 ally well illustrated by Central America, where the interpenetration 

 of North and South American faunas has proceeded in many groups 

 practically to the limit of climatic tolerance, but where no truly 

 Neotropical (South American) fresh-water fish has gotten farther 

 north than Texas or New Mexico, and none truly Nearctic (North 

 American) farther south than Nicaragua. 



There are, of course, exceptional methods by which fishes may be 

 transported. "Rains of fishes" are sufl&ciently well known and 

 authenticated to make it certain that cyclonic winds, in passing over 

 bodies of water, sometimes pick up small fishes and deposit them at a 

 distance, still alive. It is possible, too, that a fish eagle or gull might 

 drop or disgorge alive a newly caught fish after having carried it over 

 a divide between two distinct river systems. But the frequently 

 made statement that the eggs of fishes are dispersed by adhering to 

 the feet of wading birds in flight should cease to trouble zoogeographers; 

 such a method of transportation is possible, but almost no fish eggs 

 are sufficiently resistant to survive drying in the air more than a very 

 few minutes. The main fact to keep in mind is that fish distribution 

 is much more regular and understandable than it would be if these 

 unusual methods of transportation were of much importance. 



One fact that some zoogeographers who have dealt with fishes have 

 neglected is that the fishes to which the adjective "fresh-water" is 

 applied differ widely in the extent of their tolerance of salt water. 

 There are fresh-water fishes which never swun into salt water, some 

 which occasionally do but can survive it only for a short period, some 

 which habitually frequent estuaries and other brackish waters and 

 frequently enter the sea, and some which migrate back and forth 

 between river and sea either continuaUy or periodically. It is plainly 

 evident that a fish which can swim through sea water from one river 

 mouth to another is not of much use in studies of terrestrial zoo- 

 geography. 



The only fresh-water fishes that need especially concern us at 

 present are those of the first two of the categories I have just men- 

 tioned. These two categories I shall distinguish as a 'primary division 

 whose members are very strictly confined to fresh water, and a second- 

 ary division whose members are generally restricted to fresh water 



