76 THE AGE AND GKOWTH OF SALMON AND TKOUT. 



Among salmon this initial difference is very slight, with sea-trout it is 

 slightly more pronounced, and in the case of Mj0sen trout a four-year-old 

 migrant is on an average 24*7 cm., while a six-year-old is 30'2 cm. 

 (see page 64), so that the difference is thus (cf. Table XXXVIII.) not more 

 than about 100 grammes. 



The ultimate difference in weight is therefore not due to the slightly 

 greater or smaller size at migration. There must be some cause at work 

 which provides the older and slightly larger migrant with a capacity for 

 growth and for utilising its surroundings, surpassing the capacity possessed 

 by the younger and rather smaller migrant, for the former not only 

 maintains its slight start, but increases it very considerably. 



However, these examples merely give us indications and hints that 

 there are many reasons for a difference in the rate of growth, and these 

 indications are chiefly noticeable because they show us that it will only be 

 by working experimentally, and by separating and studying apart each of 



the different forms of influence which produce development, that we can 

 hope to get a clearer understanding of these growth-problems. 



It will no doubt be extremely interesting from a scientific point of view 

 to be able to go more fully into these problems, but what I consider most 

 important at present is that every additional contribution to our knowledge 

 of the causes of growth will have an important practical effect upon all 

 questions of pisciculture, that is to say questions relating to rational 

 utilisation of a given stock of a given water. 



So long as we remain in doubt regarding the reasons for difference in 

 growth, we shall also continue uncertain how to obtain the utmost benefit 

 from vigorous growth, or how to improve poor growth, or again, how to 

 gain control over development in such a way as to make these animals 

 subject to us, that is to say, by finding out the forms most suitable for 

 pisciculture. 



Questions like these can now be solved by systematic experiment, and 

 the results we have obtained will, I think, render such experiments far more 

 successful than they might otherwise have been. 



