and functional resemblances are the worst possible guide to relationship by descent. Almost all plants which grow under water have very finely divided leaves; but they are all aberrant members of the most diverse and widely separated families. They have been. compelled to acquire long, thin, waving leaflets because there was so little carbonic acid in the water where they lived that they could not extract carbon enough from it to build up a full, large, round type of leaf; exactly as all aquatic animals have muchbranched gills to catch the stray floating particles of free oxygen dissolved in the water, while ali land-animals have big internal lungs, into which the abundant free oxygen of the atmosphere pours copiously at every inhalation. Put any two distinct groups of plants or animals into exactly similar circumstances, and the chances are that they will adapt themselves to those circumstances in exactly similar ways, thus masking their original unlikeness. But if you examine their minute internal structure, you will probably still find many small points of deep-seated difference, underlying their external adaptive similarity; and these points are the important clues which aid us in discovering their real relationship to unlike groups elsewhere. Thus the humming-birds of America and the sun-birds of India are extremely similar in outer appearance, because they are both highly adapted to a flower-feeding existence; but their minute anatomy shows that the one family are modified swifts or swallows, while the other family are modified tropical fruit-eaters. Hence we are landed at last in the apparent paradox so ingeniously pointed out by Mr. A. R. Wallace, that the less functionally useful any structure may be, the greater becomes its value as a test of relationship by descent. XVII. A MOUNTAIN TARN. ONE could not find many pleasanter seats in England, or Wales either, than this big dry boulder, with a niche that seems intentionally designed to accommodate the contour of the human backbone, overlooking the calm surface and bare craggy sides of a little mountain tarn. I have come up here this morning on sport intent, to find a specimen of a peculiar species of trout, which haunts this one tiny sheet of water, and occurs nowhere else in the whole universe. Not that I am an active sportsman myself; my portion of the service is always confined to the attitude of those who stand and wait; and I don't even stand to-day, having found so comfortable a seat in the water-worn hollows of a granite boulder. But the young landscape painter from Manchester, who is making such a pretty picture of the glen from his tent close. by, can throw a fly as well as any man in Lancashire; and when I mentioned to him some time since my wish to get one of these local trout as a specimen for examination, he promised to entice one up for me on the very first morning when the light was unpropitious for sketching in the glen. To-day he dropped in after breakfast to tell me he could spare me a few hours for fishing; so here we are beside the tarn, and here is the Llyn Gwernant trout in person, flapping and floundering on the bare rock at my side-poor creature! -in its last gasps, while I am calmly prepared to watch and report upon its specific peculiarities. I have certain compunctions of my own about the morality of catching a live trout for such a purpose; but as my artist friend still continues angling for more, which, when caught, I shall doubtless eat for supper with a clear conscience, I may as well stifle my scruples now, and take notes of my trout while he is still fresh and lifelike. After all, it is just as legitimate, I suppose, to catch a fish in the interests of science as to catch it for the sake of dishing it up at supper in a tempting brown case of egg and bread-crumbs. There can be no doubt or hesitation about the right of the Llyn Gwernant trout to rank as a separate species. The marks which distinguish it from the common speckled trout of English brooks and rivers are many and undeniable. But the question how it came here is a very curious and interesting one. We have in Britain altogether some twelve kinds of trout, peculiar to our own islands; and most of them are limited, as in this case, to a single station, usually a mountain pool with only one precipitous outlet. On the old theory, which represented every species of plant or animal as the direct |