ADAPTATION 185 



The downward current and sediment, if the lat- 

 ter is not too abundant, are not obstacles suf- 

 ficient to keep an adult fish from entering fresh 

 waters. The eggs and young furnish the point 

 of attack. Among oceanic fishes we have many 

 that have pelagic eggs, others that have adhesive 

 eggs, others that have heavy cohesive eggs, others 

 that have filaments for the attachment of eggs, 

 while others harbor their eggs. 



Currents would naturally tend to carry pelagic 

 eggs into the ocean, and as far as I know only 

 one fish with pelagic eggs has succeeded in estab- 

 lishing itself in fresh water, and it, the eel, to the 

 present day, descends to the sea to deposit its 

 eggs! 



The other types of eggs of marine fishes are 

 all found in fresh waters, and it is certain that 

 in many cases the possession of eggs of one or 

 another of these sorts has enabled the fish to 

 establish itself in fresh water. Thus the major 

 adaptations were acquired by the ancestors of 

 fresh-water fishes before they were eligible to a 

 fresh-water existence. Innumerable minor adap- 

 tations to the peculiar combinations of heat, sed- 

 iment, light, etc., found in each selected locaUty, 

 have no doubt arisen in such locahties. 



When a new water area arises, selective migra- 

 tion is the method of origin of the adapted fauna. 

 The vast territory containing our North Amer- 

 ican lakes and streams north of the southern line 

 of glaciation, the area from the Arctic south to 



