Popular Science Monthly/Volume 7/May 1875/Correspondence

CORRESPONDENCE.
 

TO WHAT EXTENT IS EVOLUTION VISIBLE?

To the Editor of the Popular Science Monthly:

"It is less unphilosophical to suppose that each species has been evolved from a predecessor by a modification of its parts, than that it has suddenly started into existence out of nothing. Nor is there much weight in the remark that no man has ever witnessed such a transformation taking place."—(Draper, "History of the Conflict between Religion and Science," p. 192. "International Scientific Series," No. 12.)

There stands, in a window of my wife's sitting-room, a potted slip of geranium. To-day, I heard her remark, "I can see that this grows now, every day." In what sense is this true? By remembering its size and condition of yesterday, and comparing it with its present dimensions and conditions, an increase and change are demonstrable. It certainly has grown. But, if we were to sit down at sunrise and watch unceasingly until sunset, or by lamplight continue the vigil until sunrise again, we would in all this time have seen no enlargement of the main stem—no unfolding of a leaf-bud; still, both these changes have taken place within twenty-four hours, and, of course, in full view.

Is it not true in the same way, but far less rapidly, that the changes in animal life are constantly occurring—so gradually, that we cannot mark and measure the progress, but, like the plant, can appreciate the changes when considerably advanced? The birds, the fishes, the insects of to-day are the same that our grandparents knew. Linnaeus would recognize our white-headed eagle, if he could see one now; yet, in truth, they are not wholly the same. Just as we will realize, in the coming May, a great change in the forest-trees, then clothed in verdure, and now bare and seemingly lifeless, will not, in the life-giving spring of a coming æon, the changes not now discernible be seen, admired, and studied, by the people of that time? Taking up some of the more familiar objects about me, I have endeavored to see if there was not a possibility of detecting some trace of changes now in progress, reminding one of the changes of a growing plant.

The change now in progress in any species, say of fishes, is to become, if I am correct in my surmise, visible in fifty centuries or more. From what we can now learn of the fish, can we determine the direction of the change; can we predict its character? Our slip of geranium has to-day a small outgrowth at one side of the stem; elsewhere the bark is smooth and unbroken. If, by the microscope's aid, we study the character of the structure of the main stem, if we learn every detail of the physiology of the plant, we conclude that it is a living, healthy organism, not depending upon the leaf-bud. As a mere bud, it is not a necessity; but, as a full-blown leaf, it is. If, now, we carefully study the habits of any of our common fishes, we will find in them certain peculiar habits, which may be compared to the leaf-bud; and I believe these habits, in many cases, are only faint traces of a coming change that will expand like the opening leaf-bud, into a fully-established characteristic in the far-distant tomorrow of a coming age.

In this way—to this extent—is not evolution visible?

As an example, let me call your attention to our well-known mud-minnow (Melanura limi). This fish I have very carefully studied for several years, and seldom fail to see something peculiar in its habits, every additional hour I spend in watching them, whether in an aquarium or their native haunts. On observing the movements of some remarkably large specimens lately, in an aquarium, I was forcibly struck with the peculiar use they made of their pectoral fins. These fins, in most fishes, are kept parallel, or nearly so, with the body, and are usually thin, transparent, and with very flexible rays. These conditions, which vary in the thousands of species of fishes, do not obtain in the case of the mud-minnow The membrane is dense, the rays numerous and strong, and the fin is held at a right angle with the body when the fish is in an horizontal position, and nearly so whatever other position may be assumed. The ventral fins, likewise stiff and strongly rayed, when the fish is swimming, are not much used, but as soon as the animal comes to a rest they are spread out, and, with the pectoral fins, now stiff and motionless, they form four legs that support the body, just as a salamander does. Indeed, the likeness goes further, and the body is curved frequently when at rest, and remains so; the head turned to the right or left, the tail in the opposite direction. No one can fail to see the salamandrine appearance of this fish in this position. Now, if we follow up the habits of the fish, what unfish-like—if I may use the expression—habit other than this can we detect? Knowing its predilection for thick and muddy waters, I find it not only conceals itself in the mud during the summer, but it deeply embeds itself and regularly hibernates; and in times of drought will live, as I have determined by experiment, twenty-eight days, in stiff mud, far less moist than the usual ditch-bottom, during a dry summer. This must be recollected in connection with its salamandrine aspects when in water. Again, by experiment, I find that this minnow, out of water, will outlive all other fishes of our streams, except the eel and possibly the catfish. My experiments showed it outlived a common sunfish just 500 per cent.; a roach, 1,500 per cent.; and a catfish and mud-minnow, taken from the water together, and kept thirty-five minutes in the air, were both very sick when replaced in the aquarium. The cat-fish revived in three minutes; the minnow in eleven. This unusual ability of retaining life out of its natural conditions and surroundings is just such a peculiarity as one might look for, in this species, having once noticed the peculiarity of the fins I have mentioned. To-day, however, I noticed for the first time a movement on the part of the mud-minnows, in my aquarium, never before detected, and which made the fins more leg-like than ever. Two specimens were resting, as we have described, on the tips of the pectoral and ventral fins. Coming near them suddenly, one, and then the other, moved several "steps," i. e., they gave their fins a leg-like motion, which left ((((( faint impressions, thus (((( upon the sand. I had never seen such fin-movements on the part of this fish before, nor have I since, although for a week past I have carefully watched them.

Taking, now, into consideration the habit of resting on the tips of the fins; of giving the body a serpentine position, often maintained for many minutes; of burrowing in the mud; and able to withstand the atmosphere for a remarkable length of time; to which I think I may add walking on its fins—may we not see in all these a "leafbud," as it were, which in the far future will expand into an air-breathing, salamandrine animal?

Have we here really caught a faint glimpse of evolution?

Since the above was written, I have received and read Schmidt's "Descent and Darwinism." On page 130 he quotes from Lyell, as follows: "In a word, the movement of the inorganic world is obvious and palpable, and might be likened to the minute-hand of a clock, the progress of which can be seen and heard; whereas the fluctuations of the living creation are nearly invisible, and resemble the motion of the hour-hand of a timepiece. It is only by watching it attentively for some time, and comparing its relative position after an interval, that we can prove the reality of its motion." This quotation is the same idea far more lucidly expressed; and, had I been aware of its existence, my remarks would have remained unwritten. As it is, are they not a confirmation of my belief that evolution is, after a manner, visible, and do we not find an instance of this in the mud-minnow?

Charles C. Abbott, M. D.
Prospect Hill, Trenton, N. J., March 16, 1875.