52 THE SEA-TROUT 



out that " food is an equally important factor in bringing about colour 

 changes," and I have no doubt that sexual selection temporaply 

 affects colour also ; but I think we may neglect these latter factors 

 altogether here. The immediate point is that when the sea-trout is in the 

 sea his best protection will clearly be the unbroken mirror-like surface 

 of his body, but when he returns to fresh water a reversion to more trout- 

 like colouration will be his best protection if it serve no other purpose. 

 It may therefore be more than a mere coincidence that the trout-like 

 aspect becomes most marked as spawning time approaches when the 

 fish is in the smaller spawning tributaries. I have already shown in 

 my introduction that it is when a sea-trout has been some time in fresh 

 water that doubts arise in the angler's mind as to whether it is a sea- 

 trout or a trout. 



I understand that the " spot " is caused by local concentration of 

 the colour cells in the skin — and this may account for the almost colour- 

 less halo observable round the spots of trout — but such concentration 

 will be less necessary to the sea-trout while it is in the sea than while 

 it is in fresh water. As the spawning season approaches, the exterior 

 parts of the scales of sea-trout, as of salmon, disintegrate considerably, 

 especially in the case of male fish, while at the same time the skin 

 thickens so much that the scales become almost wholly embedded in it. 

 Therefore the spots are accentuated on the skin of the sea-trout at this 

 period and stand out boldly as they permanently do in the case of the 

 trout. 



I now arrive at the point to which these observations have tended. 

 One of the commonest " patterns " on Loch Lomond trout is a single 

 spot surrounded by others arranged in an exact pentagonal or hexagonal 

 diagram (Fig. 9), so exact that each might have been geometrically 

 drawn. Yet that is precisely the pattern of spots that commonly 

 appears on Loch Lomond sea-trout (Fig. 10), a pattern that appears 

 even halo-surrounded like a trout's spots at spawning time. It would 



