vi INTBODUCTION. 



prescribed, viz., 5'8 cm. The principal argument put forward for retaining 

 this minimum size of mesh, or, indeed, any prescribed size, was that it 

 would result in the protection of small grilse, or at least such grilse as it 

 was thought better to spare, with a view to their becoming of greater benefit 

 to the fisheries on their subsequent return from the sea as larger fish. 



This argument carried with it so much weight that it became in my 

 opinion a matter of the utmost importance to discover a suitable method of 

 acquiring the fullest possible information on the questions of the salmon's 

 age, and rate of growth, and as to its life history generally. 



My previous investigations had also frequently brought me into contact 

 with many problems relating to the marked tendency to variation shown by 

 all the varieties and species of the Salmonidae, and especially to the 

 remarkable disposition to variation possessed by all the numerous forms of 

 trout. It has long been recognised that both salmon and trout may vary 

 exceedingly in different localities, a difference which is often strikingly 

 apparent in the size of the fish. For example, in the case of salmon it was 

 well known, and it has also been proved by actual measurements, that along 

 certain parts of the coast they were larger than at others, and, further, it was 

 known that trout in certain lakes and streams attained considerable 

 dimensions ; whereas in other waters, perhaps in the immediate vicinity, 

 they barely succeeded in passing the limit at which they became sufficiently 

 large for eating. 



This remarkable and long-recognised tendency to variation on the 

 part of the fishes of the salmon species was accounted for, in various ways, 

 by different investigators. 



As I have shown at length in my introduction to " 0rret og Unglaks," 

 Linnaeus, when composing his " Systema Nature " was evidently impressed 

 by this tendency towards variation, and endeavoured to draw up a classifi- 

 cation of the separate species of the salmon family, (salmon, sea-trout, 

 burn-trout, mountain-trout, lake-trout, etc.) while later investigators, 

 inspired by the same idea, have endeavoured to establish the existence of 

 even a greater number of separate species. This practice was in fact carried 

 so far, that actually the young of the ordinary salmon were described, and 

 for a long time were known as an independent species (Salmo salmulus). 



However, the employment of a larger amount of material for the study 

 of the species resulted in a perhaps rather exaggerated disposition to reduce 

 their number, and the leading men of science at the end of last century, 

 notably the two Swedes, Smitt and Liljeborg, reduced the whole family to 

 merely two species, namely, salmon and trout. Smitt even looked upon the 

 trout as merely a variety of one main species, the Salmon (Salmo salar). All 

 varieties of salmon and trout were thus held to be sub-species of one or more 

 main species. 



