XXIX. A DRAINED FISHPOND.

I called in at my neighbour Major Warren Pauncefote’s this afternoon, and found he was just engaged in draining the fishpond by his garden. He is going to deepen it and to puddle the bottom, so as to make it fit for his boys to swim in. Meanwhile he has transferred all the larger carp to a stone trough in the back yard, where I saw at once there was not half enough water for them. I’m sure he didn’t mean to be cruel, for he is the humanest soldier that ever spitted Fuzzy-wuzzy in the Soudan on his sword; but all the same, to any one who understands the prime needs of fish-life, the condition of those poor carp was most sad to look at. As every one knows, they breathe the oxygen dissolved in water; and as hundreds of them were confined in this Black Hole of Calcutta, the amount at their disposal was, of course, quite inadequate. Some of the poor things were dead or dying, turning on their lustreless sides in the pathetically helpless way of suffocating fish; the others kept coming up every now and then to the surface, gasping for breath, and gulping down great open-jawed mouthfuls of air, to relieve their misery. No doubt the oxygen they thus swallow enters the body-cavity, and slightly assists them in aërating the blood, though much of it, also, may pass in the ordinary way through the gills, which are the regular and normal respiratory organs. It is always interesting to me, however, to watch fish when they come up thus to drink air at the surface, as goldfish often do when thoughtlessly confined in too small a glass basin; for in this instinctive act, as modern biologists now generally allow, we have the first faint beginnings of the evolution of lungs and the habit of air-breathing. Nay, more, terrestrial life itself, as a whole, depends in the last resort upon just such first feeble gaspings and gulpings. For lungs are nothing more, anatomically speaking, than developed swim-bladders, connected by a definite passage with the external air, and provided with a more or less perfect muscular mechanism for inhaling and expelling it.

In most fish, and in all the rudest types, the swim-bladder is merely a float or balloon, which can be filled with air, and compressed or expanded, so as to make the animal rise or sink at pleasure. But many fish exist in tropical ponds and shallow swamps to whom what has happened artificially to the carp in my friend’s ornamental water happens naturally every dry season; the marshy sheets in which they live evaporate altogether, and they are therefore compelled to lie dormant in the mud without food or drink for many weeks at a time. Under these peculiar circumstances, their air-bladder has gradually developed into a true lung; and, what is odder still, we possess in various countries distinct specimens at all the intermediate stages from air-bladder to lung in proportion as the ponds which they haunt become dry for longer or shorter periods. The bow-fin of the United States, for example, lives in turbid waters which do not quite dry up, but it has acquired the habit of rising to the surface every now and then, and gulping in large mouthfuls of air, which enter its swim-bladder. It does so most frequently when the water is foul, and there has been little rainfall—in other words, when there is a scarcity of oxygen. Accordingly, its air-bladder—though not yet a true lung—is spongy and cellular in structure, being adapted for aërating the blood that passes through it. The mud-fish of Queensland, again, to take a further stage, is a six-foot-long fish which inhabits loaded streams, where its gills do not suffice it for proper respiration; it has therefore altered its swim-bladder into a rudimentary lung, more advanced than the bow-fin’s, and full of air-cells, richly supplied with blood-vessels, but consisting still of a single cavity. Nevertheless, even this imperfect lung enables the mud-fish to stroll away from its native streams at night, and wander at large on dry land by means of fins which are almost legs, and which act like the sprawling limbs of certain southern lizards. In that unnatural environment it browses on green leaves, and otherwise behaves in a most unfishlike manner. Finally, to complete our rough survey, the African lepidosiren makes its home in waters which dry up completely during the hot season; and it therefore hibernates (or rather, æstivates) for months together in a cocoon of hard mud, where it breathes at its ease by means of true lungs, completely divided into lateral halves, and approaching in structure those of an air-breathing reptile.

This interesting series of living evolutionary fossils—links that are not missing—is completed for us in some ways by the frogs and toads, which recapitulate, as it were, in their own lifetime just such an ancestral developmental history. Each of them begins life as essentially a fish—that is to say, as a tadpole breathing oxygen dissolved in water, by means of gills, and possessed of no limbs for terrestrial locomotion; he ends it as essentially a full-grown land-reptile, breathing atmospheric air by means of lungs, having discarded his now needless fin-fringed tail, and possessed of jumping legs of great muscular power. And the metamorphosis he thus undergoes answers exactly to just such a drying-up of the ponds that bore him. In early spring, when the temporary puddles are full of water, the parent frogs lay their spawn by hundreds in the ancestral element; and soon the little black tadpoles—true fish of a primitive type in all but name—swarm forth and swim in seething masses in the momentary medium. But as the sun begins to dry up the water in their dwelling-place they lose their fins and gills, pass from fish to amphibians, and shortly hop ashore, provided with four legs and a pair of lungs specially adapted for direct air-breathing. There we have a marvellous piece of evolutionary magic still going on every day before our eyes, which would sound incredible to us if a man of science reported it for the first time from Central Africa or New Guinea. The frog, in short, shows us successively in his own person the self-same stages of development which the various mud-fish preserve for us in distant regions, as types of distinct and unrelated species.