THE COMMON TROUT 259 
renders highly untrustworthy the points of difference in fin- 
rays, form of gill-cover, dentition, and number of pyloric 
appendages, which have been relied on as specific distinctions. 
I possess a small lake, some six acres in extent, of exceedingly 
clear water supplied by springs. It has been formed out of an 
old marl pit, and about thirty years ago I introduced trout 
into it. The water being very rich in insect and crustacean life, 
the fish have thriven amazingly ; but, owing to the absence of 
suitable running water, they are unable to fertilise the spawn 
which forms in their ovaries at the usual season. Accordingly 
I have kept up the stock by turning in trout nearly every 
year since the beginning; with this result, that, whatever 
difference, specific or otherwise, might be apparent in the fish 
at the time they were turned in, after two seasons, at most, 
it became absolutely impossible to tell from their external 
appearance to what variety they originally belonged. Whether 
they had been small trout from a neighbouring stream, 
distinguished by conspicuous red spots, very distinct parr- 
markings, and a predominance of yellow in their colouring, or 
other small trout much darker and less shapely from a more 
distant stream, or Loch Leven trout (Salmo levenensis of 
Gunther )—all assumed when in prime condition a very silvery 
appearance, with not more difference among them than is 
apparent among sheep of the same flock. Fingerling trout 
which, if left in their native burn, would never have weighed 
a third of a pound, grow rapidly under the favourable 
conditions of this little loch to three and four pounds in 
weight. The deposit of guanin under the scales is so uniform 
as to supply a complete disguise ; the parr-marks completely 
disappear ; so also do most or all of the red spots; and I have 
taken some which, had the loch possessed any practicable 
connection with the sea, I should have pronounced at first sight 
without hesitation to be salmon-trout. 
Of peculiarities in the internal structure of these fish I 
cannot speak with confidence ; but surely it is unsafe to found 
