The Dawn of Reason | Page 9

the hottest days of summer and the coldest days of winter (that is, if there is sunlight and no ice on the water), it may be seen on the surface of ponds and streams, gyrating hither and thither in a seemingly mad and purposeless manner.
Several of these creatures will be seen at one moment floating on the water, still and motionless; the next moment they will be darting here and there over the surface of the water, their black and burnished backs shining in the sunlight like brilliant gems. Suddenly, it is "heels up and heads down," and they disappear beneath the surface, each of them carrying a bubble of air caught beneath the wing-tips; or, as the late William Hamilton Gibson expresses it, "they carry a brilliant lantern that goes gleaming like a silver streak down into the depths, for a bubble of air is caught beneath their black wing-covers, and a diamond of pure sunlight accompanies their course down among the weeds until they once more ascend to the surface."[14] This little beetle is well provided with eyes, for it has a large pair beneath its head, with which it sees all that is going on in the water below, and another pair on the sides of its head, with which it keeps a bright lookout above. That it has remarkably keen vision with the latter pair, any one who has tried to steal upon them unawares can testify.[15]
[14] William Hamilton Gibson, Sharp Eyes, p. 307.
[15] I have a distinct purpose in introducing these and other queer-eyed individuals while discussing the sense of sight. I wish to demonstrate through one or more of them the correlation of morphology, physiology, and psychology, as formulated in the first chapter of this work. This is one of the most important facts in the doctrine of evolution, and upon it is based the law of progressive psychical development from the simple manifestations of conscious determination in the lowest organisms to the most complex operations of the mind in man.
The queerest of all queer-eyed animals is, probably, the Periophthalmus, a fish inhabiting the coasts of China, Japan, India, the Malayan Archipelago, and East Africa.[16]
[16] Semper, Animal Life, p. 374 et seq.
I use the word coasts advisedly, for this strange creature when in pursuit of its prey leaves the sea and comes out on the sands, thus existing, for the greater portion of its life, in an element which, according to the general nature of things, ought to be fatal to it. The laws of evolution have, however, eminently prepared it for its peculiar mode of life, for its gill-cavities have become so enlarged that when it abandons the sea it carries in them a great quantity of water which yields up the necessary supply of oxygen.
Its locomotion has been provided for likewise, for continued use along certain lines has so developed its pectoral fins that the creature uses them as legs, and jumps along at a surprising rate of speed.
Its eyes are very large and prominent, and possess, for a fish, the peculiar faculty of looking around on all sides, hence its name, "periophthalmus," which is derived from the Greek words, [Greek: peri], around, and [Greek: ophthalmos], eye. These eyes are situated on top of the animal's head, and present a very grotesque appearance.
The favorite food of this fish is Onchidium, a naked mollusk. And, in the matter of eyes, this last-mentioned creature is itself worthy of remark. Its cephalic, or head, eyes are like those of other mollusks of its genus, and are not worthy of special mention, but its dorsal eyes, sometimes several dozen in number, are truly remarkable. These eyes, although they are very simple in structure, in type are the same as those of vertebrates, having corne?, lenses, retin?, and "blind spots." (In the vertebrate eye, the spot where the optic nerve pierces the external layer of the retina is not sensitive to light impressions; hence, it is called the "blind spot.")
When this mollusk sees periophthalmus bounding over the sands (and that it does see is beyond all question), what does it do? It contracts a thousand or so of little bladder-like cells in the skin of its back, thereby discharging a hailstorm of minute concretions in the face of its enemy. The fish, terrified and amazed by the volley, often turns aside, and the mollusk is saved. Thus we see that its dorsal eyes are of great service to onchidium.
The Greeks were, unwittingly, very near an anatomical truth when they ascribed to certain monsters, called cyclopes, only one eye apiece, which was placed in the centre of their foreheads. The cyclopean eye exists to-day in the brains of men in a rudimentary form, for in the pineal gland we find the last vestiges of that which was once