NATURAL HISTORY OF BRITISH SALMONIDZ. 159 
THE COMMON OR YELLOW TROUT 
(Salmo fario). 
This species, the mainstay and principal resource of the 
fly fisher, is so well known, and is so widely distributed over 
the whole of the British Islands, as to make any detailed de- 
scription of its appearance or habitats superfluous. Indeed, so 
far as the former is concerned, it would be practically impos- 
sible ; as its colour and shape—except in the points already 
referred to—are susceptible of infinite difference, and vary as 
much as the qualities of the waters (whether in sources or 
feeders), geological strata of the beds, and nature and quantity 
of food found in the brooks, streams, rivers, ponds, lynns, 
and lakes, where it is bred. 
Inhabiting such an infinite range of varying waters this 
diversity of colouring is, in fact, a defence given by nature to 
the trout for its preservation. Were its colour more uniform 
or unchangeable, the fish would be so plainly visible in dif- 
ferent waters or soils as to fall an obvious prey to its enemies, 
whether biped or quadruped. In fact, experiments have shown 
that the changes of colour are a question of minutes rather 
than of days and weeks. Upon its transfer from a light to a 
dark coloured vessel, or wéce versa, the hue of the trout under- 
goes an instant alteration, and in a very short time assimilates 
itself more or less perfectly to that of its new domicile. Thus, 
for instance, the trout of Lynn Ogwin, almost the whole bottom 
of which is formed of grass, have, when first caught, a brilliant 
emerald gloss over their golden and yellow tints ; and although 
the waters are of the utmost clearness and the lake swarming 
with fish, I was never able in any one instance to distinguish 
these from their surrounding green. Again on the Spean 
Water, Inverness, there are several small tarns in which I have 
frequently taken fish almost the colour of ink; yet these 
tarns actually join the Spean, where many of the trout are of a 
fine rich yellow—the cause of the difference being that the 
