THE LIFE-HISTORIES OF FISHES 171 



vary with each species and with the locality 

 studied. As an instance of a life-history differing 

 from that of the plaice I may cite that of the 

 fresh-water eel ; here the adult descends from the 

 rivers into the sea to spawn, and the young eels 

 ascend the rivers as " elvers " to feed and grow to 

 reproductive maturity. Opposed to such a life- 

 history is that of the salmon and some allied forms. 

 These fishes ascend the rivers to spawn, and descend 

 again into the sea, where they grow to maturity. 

 Such migratory periods as that of the plaice, eels, 

 and salmon are comparatively simple ; but there 

 are others which are not yet understood. So much 

 has been written about the migrations of the 

 herring that I hesitate to mention the subject here. 

 The old theory had nothing to commend it but its 

 simplicity. It was supposed that the fish had its 

 home somewhere in the northern seas, and that 

 regularly every season the shoals migrated south- 

 ward in a mass. This explained why the fishing 

 season is generally a little later the farther down 

 the coast, starting at (say) Wick, we go. But it 

 does not explain why in the summer and later 

 herring fisheries the fish caught always have ripe 

 reproductive organs. If the shoals which are 

 fished on the north-east coast of Scotland in 

 summer are the same that visit the south-east 

 coast of England in late autumn, then it is difficult 

 to see how in both cases the fish can be spawning. 

 Many observations have, in fact, led to the belief 



