"_Physicus._--I think I understand you, that the memory of the danger and pain does not last long, unless there is a permanent sensation with which it can remain associated,--such as the station of the trout; and that the recollection of the mere form of the artificial fly, without this association, is evanescent. "_Ornither._--You are diving into metaphysics; yet, I think, in fowling, I have observed that the memory of birds is local. A woodcock that has been much shot at and scared in a particular wood, runs to the side where he has usually escaped, the moment he hears the dogs; but if driven into a new wood, he seems to lose his acquired habits of caution, and becomes stupid." In alluding to the migrations of fishes, _Physicus_ observes, "That he has always considered that the two great sources of change of place of animals, was the providing of food for themselves, and resting-places and food for their young. The great supposed migrations of herrings from the poles to the temperate zone appear to be only the approach of successive shoals from deep to shallow water, for the purpose of spawning. The migrations of salmon and trout are evidently for the purpose of depositing their ova, or of finding food after they have spawned." In explaining the circumstances which render a migration into shallow water necessary for the developement of the ova, Davy evidently bears in mind the result of his very first experiment.[111] [111] Vol. i. page 40. "Carp, perch, and pike, deposit their ova in still water, in spring and summer, _when it is supplied with air by the growth of vegetables; and it is to the leaves of plants, which afford a continual supply of oxygen to the water, that the impregnated eggs usually adhere_." Again: "Fish in spawning-time always approach great shallows, or shores covered with weeds, _which, in the process of their growth, under the influence of the sunshine, constantly supply pure air to the water in contact with them_." The following passage will afford a good specimen of the familiar dialogue, while it will convey to the reader some curious facts connected with the influence of sunshine. "_Halieus._--Well, gentlemen, what sport? "_Poietes._--The fish are rising every where; but though we have been throwing over them with all our skill for a quarter of an hour, yet not a single one will take; and I am afraid we shall return to breakfast without our prey. "_Halieus._--I will try; but I shall go to the other side, where I see a very large fish rising--There!--I have him at the very first throw--Land this fish, and put him into the well. Now, I have another; and I have no doubt I could take half-a-dozen in this very place, where you have been so long in fishing without success. "_Physicus._--You must have a different fly; or, have you some unguent or charm to tempt the fish?