I042 FRESH-WATER BIOLOGY 



such as the lampreys and salmon, which make very elaborate 

 preparations to produce their young at a great distance from 

 their range, the eels never regain their range, and probably all 

 die after the first reproductive period. It is not improbable that 

 in some landlocked lakes eels mature and reproduce in fresh water, 

 but no eels with ripe eggs, nor eggs, nor larval eels, have been found 

 in fresh water. 



Homes of Fishes. — Emphasis has been laid on the fact that 

 the ultimate fate of all fresh water is locomotion, and that usually 

 currents exist between fresh-water lakes and the ocean. All fresh- 

 water fishes are adapted to this condition and make provision to 

 anchor their eggs or give birth to living young. There is but one 

 fresh-water fish known to me that has pelagic eggs, the eel. It 

 has not become adapted to rear its yoxmg in fresh water, but enters 

 the sea before the reproductive period. This suggests that the 

 adaptation of fresh-water fishes to resist currents did not arise 

 after they had entered the fresh water, but that such oceanic 

 candidates for fresh-water existence as had eggs adapted to resist 

 the currents gained a permanent lodgment; while, on the contrary, 

 none of those with pelagic eggs have been able to estabhsh perma- 

 nent homes in fresh waters. All anadromous and fresh- water 

 fishes either have eggs heavier than water which lodge in gravel, or 

 produce attachable eggs. Many marine fishes have pelagic eggs, 

 and none of these have become permanent residents in fresh water. 

 Others have adhesive eggs, that at the moment of being laid will 

 adhere to foreign substances; others have cohesive eggs that will 

 become attached to each other, but not to foreign substances. Of 

 the adhesive eggs some are simply sticky all over and others have 

 mushroom-shaped processes that have sticky heads (stickleback). 

 The eggs of still other fishes have filaments that coil about foreign 

 substances. All of these types are found in fresh waters. The 

 fundamental adaptation, that to flowing water, was acquired by 

 the ancestors of fresh-water fishes before they were able to leave 

 the ocean. 



Along with this adaptation against currents, we have in the 

 fresh-water fishes elaborate brooding habits that in part, at least, 

 are an adaptation to another fresh-water condition, i.e., the settling 



