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
