HAL.—If so, they have been altered in a succession of generations. The common trouts of this lake have stomachs like other trouts, which never, as far as my experience has gone, contain shell-fish; but of the gillaroo trout, I have caught with a fly some not longer than my finger, which have had as perfect a hard stomach as the larger ones, with the coats as thick in proportion, and the same shells within; so that this animal is at least _now_ a distinct species, and is a sort of link between the trout and char, which has a stomach of the same kind with the gillaroo, but not quite so thick, and which feeds at the bottom in the same way. I have often looked in the lakes abroad for gillaroo trout, and never found one. In a small lake at the foot of the Crest of the Brenner, above 4000 feet above the level of the sea, I once caught some trout, which, from their thickness and red spots, I suspected were gillaroo, but on opening the stomach I found I was mistaken; it had no particular thickness, and was filled with grasshoppers: but there were _char_, which fed on _shell-fish_, in the same lake. POIET.—Are water-flies found on all rivers? HAL.—This is a question which I find it impossible to answer; yet from my own experience I should suppose, that in all the habitable parts of the globe certain water-flies exist wherever there is running water. Even in the most ardent temperature, gnats and musquitoes are found, which lay their congeries of eggs on the water, which, when hatched, become first worms, afterwards small shrimp-like aurelia, and lastly flies. There are a great number of the largest species of these flies on stagnant waters and lakes, which form a part of the food of various fishes, principally of the carp kind: but the true fisherman’s flies,—those which are imitated in our art, principally belong to the northern, or at least temperate part of Europe, and I believe are nowhere more abundant than in England. It appears to me, that since I have been a fisherman, which is now the best part of half a century, I have observed in some rivers where I have been accustomed to fish habitually, a diminution of the numbers of flies. There were always some seasons in which the temperature was favourable to a quantity of fly; for instance, fine warm days in spring for the grannam, or brown fly; and like days in May and June for the alder-fly, May-fly, and stone-fly; but I should say that within these last twenty years I have observed a general diminution of the spring and autumnal flies, except in those rivers which are fed from sources that run from chalk, and which are perennial—such as the Wandle, and the Hampshire and Buckinghamshire rivers; in these streams the temperature is more uniform, and the quantity of water does not vary much. I attribute the change of the quantity of flies in the rivers to the cultivation of the country. Most of the bogs or marshes which fed many considerable streams are drained;