40 BRITISH FRESHWATER FISHES 



and by the form of the caudal peduncle and anal 

 fin. 



The fact that Salmon nearly always return to the 

 rivers from which they came may perhaps be 

 explained on the supposition that they never really 

 lose touch with their river, that the particular 

 feeding-grounds to which the fish of any stream 

 resort are, as it were, within its sphere of influence. 

 This would account for the peculiarities of form and 

 size associated with Salmon from different rivers, 

 for the area of the feeding-grounds would be to 

 some extent proportional to the size of the stream. 

 Thus Salmon from a small river would seldom grow 

 large, and, if they did, might make their way into the 

 nearest large river instead of ascending their own. 



The homing instinct is a powerful one, but may 

 be broken through ; every year a few Salmon appear 

 off the mouth of the Thames and, if the water were 

 pure enough, would ascend and reconvert it into a 

 Salmon river, which will never be the case under 

 present conditions, however many parr may be 

 placed in the upper waters. Mr. Holt instances the 

 case of the Ballysodan River, which became a Salmon 

 river by the successful engineering of a pass on the 

 previously inaccessible fall at its mouth, as effectually 

 disposing of the theory that the Salmon chooses the 

 particular river from which it came, and no other. 



The normal life-history of the Salmon has now 

 been described, and it may perhaps be useful to 

 offer definitions of the names applied to the fish at 

 various stages ; these are — 



Alevins. — Fry with yolk-sac attached, found on 

 spawning-beds in the early spring. 



