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 

 Giinther) — 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 



