THE SALMON 189 



that salmon leaving the sea are not always acting under the 

 reproductive impulse. 



This is no matter for dogma, but the explanation of 

 the periodical movement of salmon which most nearly 

 corresponds with my own observation is as follows. 



Like many of the lower vertebrate animals, especially 

 fish, salmon manifest intermittent periods of appetite during 

 which growth proceeds, and of abstinence, during which 

 growth is suspended.* The salmon feeds and grows without 

 intermission through the stages of alevin, parr, and smolt, 

 and continues to do so in the sea in its transition to the 

 grilse stage. Under the stimulus of unlimited nutrition, the 

 fish rapidly increases in bulk, until the time comes when its 

 tissues are so fully stored with fats and proteids, that no more 

 nourishment can be assimilated, appetite ceases, and the fish 

 returns to its native river. There is no evidence to connect 

 the return of grilse in May, five or six months before the 

 spawning season, with the sexual impulse. It appears probable 

 that, the purpose with which the fish visited the sea having 

 been accomplished, it simply returns home, i.e.^ to its native 

 fresh water, which is its most congenial environment. After 

 an uncertain period, appetite may return, and the fish obeys 

 the impulse to seek again the salt water, teeming with various 

 forms of life offering an abundance which cannot be found 

 in the river. 



* As an extreme instance of periodical fasting and arrest of growth, 

 the so-called lepidosiren {Protopterus annectens) may be cited. This fish, 

 which belongs to the Ganoid Order, abounds in many rivers of tropical 

 Africa. These rivers usually disappear in the dry season, when the fish 

 bury themselves in the mud, lining their resting-place with a protecting 

 case of mucus. The mud- balls thus formed may be dug out and have 

 been brought to Europe, where the fish inside can be released by 

 immersing their domicile in tepid water. Here it is obvious that there 

 must be absolute abstinence from food for long periods, and the rigid 

 envelope renders the corresponding cessation of growth a necessary 

 consequence of the situation. 



