74 THE AGE AND GROWTH OF SALMON AND TEOUT. 



It is possible, therefore, to divide the fish into two classes, according 

 to their manner of growth. 



1. Fish with uniform development (trout-like growth). 



2. Fish with altered development (salmon-like growth). 



A classification of this sort is perfectly feasible, and it will be both 

 useful and instructive, inasmuch as it will always convey valuable biological 

 information as to whether the fish had spent all its existence amidst 

 uniform conditions, or has been subjected to very considerable alterations 

 in its surroundings. 



Our investigations, then, have won for us this advantage : that the old 

 question regarding the variability of the salmonidae may be reduced to a 

 mere matter of age and growth, a difference that we are in every case able 

 to throw light upon. 



When, however, we proceed to ask about the causes of this difference, 

 we find ourselves often at a loss. One may naturally enough conclude 

 that the extraordinary alteration in development which we observe in the 

 growth -history of salmon, sea-trout, and several other sorts of trout, must 

 be closely connected with change of residence and access to better food. 

 But it would be short-sighted on our part to assume that the quantity of 

 food is the only deciding kind of influence that regulates the course and 

 rapidity of development. Growth is by no means a simple phenomenon. 

 On the contrary, it is the product of influences of many kinds. Undoubtedly, 

 as I have previously mentioned, food plays an extremely important part, 

 and possibly the most important. Still, even in this particular factor there 

 are many variations, and there are, besides, quite a number of other 

 influences whose significance cannot be disregarded. 



We can, for instance, mention quality of food, opportunity of obtaining 

 appropriate food, variation in temperature, duration of winter (with which 

 is connected the duration of the period of growth), the chemical composition 

 of the water, the influence of topographical conditions, and the number of 

 the stock. Last of all, there is the old question of race. 



Even if we can demonstrate that the old problem regarding variability, 

 in so far as it is expressed by size, may be reduced to differences of age and 

 growth, we are still faced with the possibility that underlying the great 

 plasticity in growth which we have observed, there may be concealed a 

 difference in race, a hereditary difference, showing itself by a more or less 

 pronounced tendency to rapid development, or, if one may say so, a greed 

 of greater or lesser degree, by which is implied a greater or less power of 

 absorbing and utilising food or conditions of existence. 



Many other influences might be mentioned, but it is as well to confess 

 that we are, on the whole, not in a position to judge of their importance, or 

 to assert positively what part they play. 



