By DAVID STARR JORDAN.
Long ago, in the old Devonian times, when life was very leisurely, all the beasts and people that there were lived in the sea together. The air was dull and murky on the land. It was so light that it gave no support to the body, and so those that ventured about in it had to lie prone on the ground all the time wherever they went. So they preferred to stay in the water, where motion is much easier. Then, too, water is so much better to breathe than air, if one has gills fitted for it! He has only to open his mouth and the water rushes in. Then he has only to shut his mouth and the water rushes out backward, bathing his gills on the way. Thus, the air dissolved in the water purifies all the little drops of blood that run up and back through the slender tubes of which the gills are made.
But in those days, besides the gills, some of the beasts of the sea had also a sac in the throat above the stomach in which they could stow away air which they took from the atmosphere itself. This served them in good stead when they were in crowded places, in which the air dissolved in the water would fail them.
And those which were so provided used to venture farther and farther out of the water, pushing their way heavily on the ground. And those which could put forth most effort survived, until at last their descendants were able to maintain themselves on the land altogether. These gave rise to the races of reptiles and birds and mammals, the ancestors of all the land beasts that you know, as well as men and women and all the monkey people. But it was very long ago when this happened, and because these ancestors came finally out of the water they have no part in the story I am trying to tell to-day.
Those that remained in the water grew more and more contented with their condition. Because the medium in which they lived was as heavy as their bodies, they swam without much effort, and effort not being needed, it was not put forth. As there was food enough in the water, they did not need to go on land. As they did not go on land, they did not use their lungs for breathing, the air sac gradually shrank away, or was used for some other purpose, and all the parts of the body became adjusted for life in water, as those of their cousins who left the sea became fitted for the life in air. Being now fishes for good, all the progress since then has made them with each succeeding century more and more decidedly fishy.
And because they are fishes they are contented to live in little brooks, which would not satisfy you and me at all. But our ancestors in the early days were more ambitious, and by struggle and effort won what seems to us a larger heritage.
So it happened one spring when the ice melted out from some little brook that flows down from somebody's hills somewhere toward some river that sets toward the Mississippi, the little fishes began to run.
And first of all came the lampreys, but they hardly count as fishes, for they have yet to learn the first principles of fishiness. A fish is a creature whose arms and legs are developed as fins, having cartilaginous rays spreading out fanlike to form an oar for swimming. But the lamprey has no trace of arm or leg, not even a bone or cartilage hidden under the skin. And its ancestors never had any limbs at all, for the earliest lamprey embryo shows no traces of them. If the ancestors ever had limbs, the descendants would never quite forget it. Some little trace would be kept by the clinging force of heredity, and at some time or another this rudiment would appear. And the lower jaw they lack too, for that is really another pair of limbs joined together in front—as it were, a pair of short hands clasped together and never unlocked.
But though the lampreys have no limbs and no jaws and are not fishes anyhow, they do not know the difference, and come up the brook in the spring, rushing up the rapids, swirling about in the eddies, just as if they were real fishes and owned the brook themselves. They are long, slender, and slippery, shaped like eels, without any scales and with only a little fin, and that along the back and tail, an outgrowth from the vertebral column. The vertebral column itself is limp and soft, the vertebræ only imperfectly formed and made of soft cartilage. In front the lamprey seems to be cut off short, but if we look carefully we see that the body ends in a round disk of a mouth, and that this disk is beset by rows of sharp teeth. A row of the sharpest of these is placed on the tongue, and two of these are above the gullet, for the tongue to scrape against them. And the rest are all blunt and are scattered over the surface of the mouth, which has no lips nor jaws, but is surrounded by a belt of fringes. When the lamprey is hungry he puts his mouth against the side of some fish, exhausts the water between, and then the pressure of the outside water holds him there tightly. When this is done, the fish swims away and the lamprey rides with it, giving no thought to where he is going, but all the while scraping away the flesh with his rasplike teeth. When he has filed off enough fish flesh to satisfy his hunger he lets go, and goes off about his business. The fish, who does not know what hurt him, goes off to get well if he can. Usually he can not, for the water of the brook is full of the germs of little toadstool-like plants, and these fasten themselves on the fish's wounds and make them bigger and bigger, until at last the cavity of the abdomen is pierced and little creatures of many kinds, plant and animal, go in there and plunder all this fish's internal organs, to carry them away for their own purposes.
But when the lampreys come up the April brook it is not to feed on fishes, nor is it to feed at all. Nature is insistent that the race should be kept up, and every animal is compelled to attend to the needs of the species, even though it be at the sacrifice of all else. If she were not so, the earth and the seas would be depopulated, and this is a contingency toward which Nature has never looked.