INTRODUCTORY 35 



to explain the presence of trout above the Falls of Clyde by assuming 



that the stock ascended the river before subsequent elevation of the 



land and denudation combined slowly formed the now impassable 



barriers of Stonebyres, Corra Linn and Bonington Falls, and so isolated 



the trout of the upper waters. The trout above these falls are 



uniformly yellow, beautifully marked with brown spots and are, whether 



old or youn^, as unlike any fresh-run, silvery sea-trout in general 



superficial appearance as it is possible for any trout to be. On the other 



hand, for thousands of years migratory trout must have had free access 



to the Loch Lomond basin, which, in a comparatively recent geological 



period, has been a sea-loch, and is now only 26.9 feet above mean 



sea-level, being connected by the short even-flowing Leven with the 



Clyde estuary. But in contrast with the trout of the upper reaches of 



the Clyde, hardly any trout are so likely to be mistaken by the angler 



for sea-trout as are the trout of Loch Lt)mond. Indeed, in the later 



months, when sea-trout which have been in the loch for some weeks 



have lost their silvery sheen, they approximate in appearance so closely 



to the native non-migratory trout that it is difficult, even for experienced 



anglers, to distinguish between them. In spring it is a practical 



impossibility to distinguish young trout of two or three years old from 



sea-trout parr of equal age. I have been able to discover no features, 



beyond some small presumptive differences of colouration, by which 



one kind of fish can be distinguished from the other, and boatmen with 



life-long experience will seldom undertake to say with any assurance 



which is which. 



The inference would seem to be legitimate that once the barrier on 

 the Clyde reached a certain height, and trout, though many might 

 descend, were barred from returning, the stock in the upper waters began 

 to revert to the original type of fresh-water trout, or at least to develop 

 along lines of colouration adapted to its isolated fresh-water environ- 

 ment, and has for centuries been so developing ; while, as regards the 



