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; and the consequence is, that they are more likely to be affected by severe droughts and great floods—the first killing, and the second washing away the larvæ and aurelias. May-flies thirty years ago were abundant in the upper part of the Teme river in Herefordshire, where it receives the Clun: they are now rarely seen. Most of the rivers of that part of England, as well as of the west, with the exception of those that rise in the still uncultivated parts of Dartmoor and Exmoor, are rapid and unfordable torrents after rain, and in dry summers little more than scanty rills; and Exmoor and Dartmoor, almost the only considerable remains of those moist, spongy, or peaty soils, which once covered the greatest part of the high lands of England, are becoming cultivated, and their sources will gradually gain the same character as those of our midland and highly-improved counties. I cannot give you an idea of the effects of peat mosses and grassy marshes on the water thrown down from the atmosphere, better, than by comparing their effects to those of roofs of houses of thatched straw, as contrasted with roofs of slate, on a shower of rain. The slate begins to drop immediately, and sends down what it receives in a rapid torrent, and is dry soon after the shower is over. From the sponge-like roof of thatch, on the contrary, it is long before the water drops; but it continues dropping and wet for hours after the shower is over and the slate dry.
Poiet.—You spoke just now of the gillaroo trout, as belonging only to Ireland. I can, however, hardly bring myself to believe, that such a fish is not to be found elsewhere. For lakes with shell-fish and char are common in various parts of Europe, and as the gillaroo trout is congenerous, it ought to exist both in Scotland and the Alpine countries.
Hal.—It is not possible from analogies of this kind to draw certain inferences. Subterraneous cavities and subterranean waters are common in various countries, yet the Proteus Anguinus is only found in two places in Carniola—at Adelsburg and Sittich. As I mentioned before, I have never yet met with a gillaroo trout except in Ireland. It is true, it is only lately that I have had my attention directed to this subject, and other fishermen or naturalists may be more fortunate.
Poiet.—Have you ever observed any other varieties of the trout kind, which may be considered as, like the gillaroo, forming a distinct species?
Hal.—I think the par, samlet, or brandling, common to most of our rivers, which communicate with the sea, has a claim to be considered a distinct species; yet the history of this fish is so obscure, and so little understood, that, perhaps, I ought not to venture to give an account of it. But in doing so, you will consider me as rather asking for new information, than as attempting a satisfactory view of this little animal.
Orn.—I have seen this fish in the rivers of Wales and Herefordshire, and have heard it asserted, on what appeared to me good authority, that it was a mule,—the offspring of a trout and a salmon.
Hal.—This opinion, I know, has been supported by the fact, that it is found only in streams, which are occasionally visited by salmon; yet I know no direct evidence in favour of the opinion, and I should think it much more probable, if it be a mixed race, that it is produced by the sea trout and common trout. In a small river, which runs into the Moy, near Ballina in Ireland, I once caught in October a great number of small sea trout, which were generally about half-a-pound in weight, and were all males; and unless it be supposed, that the females were in the river likewise, and would not take the fly, these fish, in which the spermatic system was fully developed, could only have impregnated the ova of the common river trout. The sea trout and river trout are, indeed, so like each other in character, that such a mixture seems exceedingly probable; but I know no reason why such mules should always continue small, except that it may be a mark of imperfection. The only difference between the par and common small trout is in the colours, and its possessing one or two spines more in the pectoral fin. The par has large blue or olive bluish marks on the sides, as if they had been made by the impression of the fingers of a hand; and hence the fish is called in some places fingerling. The river and sea trout seem capable of changing permanently their places of residence; and sea trout appear often to become river trout. In this case they lose their silvery colour, and gain more spots; and in their offspring these changes are more distinct. Fish, likewise, which are ill-fed remain small; and pars are exceedingly numerous in those rivers where they are found, which are never separated from the sea by impassable falls; from which I think it possible that they are produced by a cross between sea and river trout. The varieties of the common trout are almost infinite; from the great lake trout, which weighs above 60 or 70lbs., to the trouts of the little mountain brook or small mountain lake, or tarn, which is scarcely larger than the finger. The smallest trout spawn nearly at the same time with the larger ones, and their ova are of the same size; but in the large trout there are tens of thousands, and in the small one rarely as many as forty,—often from ten to forty. So that in the physical constitution of these animals, their production is diminished as their food is small in quantity; and it is remarkable, that the ova of the large and beautiful species which exist in certain lakes, and which seem always to associate together, appear to produce offspring, which, in colour, form, and power of growth and reproduction, resemble the parent fishes; and they generally choose the same river for their spawning. Thus, in the lake of Guarda, the Benacus of the ancients, the magnificent trout, or Salmo fario, which in colour and appearance is like a fresh run salmon, spawns in the river at Riva, beginning to run up for that purpose in June, and continuing to do so all the summer; and this river is fed by streams from snow and glaciers in the Tyrol, and is generally foul: whilst the small spotted common trouts, which are likewise found in this lake, go into the small brooks, which have their sources not far off, and in which, it is probable, they were originally bred. I have seen taken in the same net small fish of both these varieties which were as marked as possible in their characters:—one silvery, like a young salmon, blue on the back, and with small black spots only; the other, with yellow belly and red spots, and an olive-coloured back. I have made similar observations in other lakes, particularly in that of the Tarun near Gmunden, and likewise at Loch Neah in Ireland. Indeed, considering the sea trout as the type of the species trout, I think all the other true trouts may not improperly be considered as varieties, where the differences of food and of habits have occasioned, in a long course of ages, differences of shape and colours, transmitted to offspring in the same manner as in the variety of dogs, which may all be referred to one primitive type.[[4]]
Phys.—I am somewhat amused at your idea of the change produced in the species of trout by the formation of particular characters by particular accidents, and their hereditary transmission. It reminds me of the ingenious but somewhat unsound views of Darwin on the same subject.
Hal.—I will not allow you to assimilate my views to those of an author, who, however ingenious, is far too speculative; whose poetry has always appeared to me weak philosophy, and his philosophy indifferent poetry: and to whom I have been often accustomed to apply Blumenbach’s saying, that there were many things new and many things true in his doctrines; but that what was new was not true, and what was true was not new.