172 . FISH AS FA THERS. 



for instance, is by descent a trout, and in the parr stage 

 he is even now almost indistinguishable from many 

 kinds of river-trout that never migrate seaward at all. 

 But at some remote period, the ancestors of the true 

 salmon took to going down to the great deep in search of 

 food, and being large and active fish, found much more 

 to eat in the salt water than ever they had discovered in 

 their native streams. So they settled permanently in 

 their new home, as far as their own lives went at least ; 

 though they found the tender young could not stand the 

 brine that did no harm to the tougher constitutions of 

 the elders. No doubt the change was made gradually, 

 a bit at a time, through the brackish water, the species 

 getting further and further seaward down bays and 

 estuaries with successive generations, but always re- 

 turning to spawn in its native river, as all well-behaved 

 salmon do to the present moment. At last, the habit 

 hardened into an organic instinct, and nowadays the 

 young salmon hatch out like their fathers as parr in 

 fresh water, then go to the sea in the grilse stage and 

 grow enormously, and. finally return as full-grown salmon 

 to spawn and breed in their particular birthplace. 



Exactly the opposite fate has happened to the eels. 

 The salmonoids as a family are freshwater fish, and by 

 far the greater number of kinds trout, char, whitefish, 

 grayling, pollan, vendace, gwyniad, and so forth are in- 

 habitants of lakes, steams, ponds, and rivers, only a very 

 small number having taken permanently or temporarily to a 

 marine residence. But the eels, as a family, are a salt- 

 water group, most of their allies, like the congers and 

 muraenas, being exclusively confined to the sea, and only 



