SALMON TRIBE. 



499 



line between a species and a variety ; and the question is accordingly of no very 

 great importance one way or another. Some of the characters distinguishing the 

 salmon from the trout have been already indicated on p. 494 ; and it will suffice to 

 note very shortly some of the reasons given by Day for regarding all the British 

 trout as referable to a single species. It is well known that sea-trout — as 

 represented not only by the typical form, but likewise by the so-called sewen 

 (S. cambricus) of the Welsh rivers — are silvery in colour, with black spots during 

 their sojourn in the sea ; when, however, they enter the rivers for the purpose of 

 spawning, an orange margin appears on the upper and lower edges of the caudal, 



MAY-TROUT AND HDCHO (^ nat. size). 



and likewise on the fatty, fin ; while spots of the same colour show themselves on 

 the body. On the other hand, the nonmigratory forms may be arranged under 

 two types of coloration, some loch-trout (which may have been originally migratory, 

 but are now landlocked) being mainly silvery during the smolt-stage, and subse- 

 quently golden and spotted ; while the estuarine, lake, and river-trout are all 

 golden, with purplish reflections, and more or less fully marked with black and 

 vermilion spots. It appears, indeed, that a long residence in fresh water generally 

 leads to the disappearance of the silvery sheen characteristic of the salmonoids 

 while in the sea (and which is probably their primitive type of coloration), and to 

 the promotion of colour. As a partially transitional type between sea-trout and 



