by Guy Murchie
The chameleon's eye, for instance, is not only actively bombarded by photons of light out of the sun and by the glances of other eyes upon it from outside, but it possesses two kinds of passive vision through one of which it sees the world around it (via those same photons) and, through the other, its own body, in effect visualizing what is seen by those same outside eyes that look upon it, then influencing its body to shift the color pigment of its skin accordingly. This is proved by the fact that, if you blindfold a chameleon, he can no longer camouflage himself by matching a changing background. The songbird likewise has a sense of singing as well as of listening, the two being intimately attuned. And the skunk possesses a perceptive sense of broadcasting scent as well as of smelling it, even though these involve opposite ends of his body. Sonar and radar are only two out of many sense systems that must send before they can receive. Next come the vicarious senses, the percepts borrowed or handed on from others, such as a watchdog whose nose sniffs an approaching stranger so his master's ears will hear the warning bark and his master's eyes look out for whoever is coming. And these include the tempo of the cricket's chirp, the droop angle of the rhododendron's leaves and the space-density of animals in a flock that tell you the temperature like a living thermometer.
Then there are the artificial senses, starting with the stick, the shovel, and calendar, the clock, the weathervane, the abacus, the lightning rod, the telescope and the smoke signal, and reaching to the camera, the speedometer, the telephone, the taped color TV program, the pacemaker, bionics and the computer. We don't count these vicarious and artificial systems in our main roster of senses, however, because they are uncountably numerous, particularly the artificial ones, evolving and multiplying constantly.
SIGHT
How far science has taken us can be suggested by the surmise of Empedokles in the fifth century B.C. that "perception is chiefly in the blood, especially near the heart, our organ of consciousness. For we think mainly with our blood in which, of all parts of the body, the elements are most completely mingled." Other ancient philosophers wondered whether a bell might be consumed by frequent ringing or musk all used up on a journey because of perfuming a hundred miles of countryside. And sight, according to a theory still prevalent in the seventeenth century, was produced by light inside the eye flashing forth to illumine whatever was seen, like the glance cast by a man at an attractive woman, which seems to involve something moving actively to her, something positive she can feel. But the man's active glance is now understood to be positive mainly in a mental or spiritual sense, while the most significant material involved (a beam of photons) originates in the sun or a lamp and is reflected primarily from the woman into the man's eye rather than the other way. Indeed, as some people who put on dark glasses to aid their vision seem not yet to have learned, the eye is a two-way organ, a "window of the soul" that actually may serve its owner better by being looked into than out of.
Yes, the eye is the prime achievement of sensory evolution on Earth, an ellipsoid of optical revolution, whose chief source of energy is 93 million miles away in the sun! There remains a good deal of mystery about how it evolved, but it has become probably the most important and widely used of all sense organs, its rudimentary forms being found among the "lenses" of leaves that are vital to photosynthesis and even among translucent and transparent minerals, where its significance is still but dimly understood.
The vision of simple forms of marine animals like the one-celled flagellate Euglena is similar to that of plants, consisting primarily of light-sensitive eyespots that are microscopic and may have evolved directly from chloroplasts, the organs of photosynthesis that aim the leaf toward the sun (which gives it energy too) or, should the glare get too hot, turn it toward the shade. Light sensitivity, however, may be a general attribute (or at least a potentiality) of all cells, for experiments show that a flash of light elicits a measurable response in the brain cells of insects, in the heart muscles of certain snails and in numerous other muscle cells, even some in mammalian skin.
When the ocular lens finally evolved, using various conglomerations of translucent cells, nature experimented rather lavishly, sometimes sprouting the lens on the body's outer surface (as in the scorpion), sometimes at the ends of stringy arms (as in the stylophthalmus),
sometimes sinking it in a pit (as in the limpet), sometimes enveloping it completely in protoplasm (as in the snail), at the same time trying out all sorts of eyes of simple, ingenious, weird, radical and multiple function. Scallops, for example, developed as many as 200 eyespots (each with its lens and protoretina), which respond to darkness and are vital to these mollusks as they swim about by jet propulsion, an activity in which they can easily be caught and eaten should they fail to notice the occasional shadow made by an approaching predator. But the first camera-type eye to evolve seems to have been that of the pinhead-sized Copilia, probably the smallest animal capable of seeing actual images of things around it. It has a transparent body with two sets of double, microscopic lenses geared to L-shaped "retinas" that sweep to and fro like scanning radar dishes, flashing slow signals through optic nerves to a microscopic brain found able to reconstruct a complete image after each scan, which takes at least a fifth of a second and sometimes ten times that.
The next kind of eye to evolve on Earth seems to have been the cluster of eyespots that became the compound eye, common to a few marine animals and many insects. The supreme example is the compound eye of the male horsefly which arrayed about 7000 lenses in crystalline rows like a microscopic honeycomb. Unlike human eye lenses, these are rigidly fixed infocus and therefore rarely form sharp images or distinguish a bee from a pebble, but they register the movement of any visible object passing from lens to lens with such efficiency that a fly may accurately judge the speed of anything from the minute hand on a watch to a swooping bird or a lashing tail, a perception which often enables it to escape in time. This also explains why honeybees are particularly attracted to flowers swaying across their line of sight.
In the seemingly endless course of earthly evolution, eyes have produced bewilderingly varied answers to the visual needs of their owners, among which a number of general solutions have emerged. A swift animal moving about a large or complex environ, for instance, is apt to have big eyes (relative to body size) for wide, sweeping vision: the squirrel, the dragonfly, the eagle. A nocturnal creature has even bigger light-receptive ones, almost absurdly conspicuous, as in the owl and the tarsier. But an underground animal, an internal parasite or a cave dweller, has small eyes or none at all: the mole, the worm. And various water animals have periscopic ones: the hippo, the frog, the fiddler crab or the stylophthalmus larva who waves his about like arms.
As to specialized optical systems, predatory animals who pursue and catch elusive prey of course require keen forward vision, with at least two eyes coordinating stereoscopically, while the quarry, habitually in danger from unexpected directions, naturally have bulging eyes on both sides of their heads to ensure 360° vision. Which is why the prowling cat and the owl look straight ahead and the timid rabbit and the deer see sideways and all around, even backward. The owl in fact has eyes fixed in their sockets like headlights, so he has to turn his whole head to shift his gaze, the doing of which for millions of years has evolved a neck so flexible it can swivel in a tenth of a second more than a full circle: approximately 400° And most game animals, to whom it is often more vital to know who is behind than before them, can see their own tails and trails without turning their heads.
The much-hunted woodcock literally has his eyes in the back of his head (just abaft the center line) so that, when he is probing the ground in front of him for grubs with his long, flexible bill (where sight is hardly needed), he has excellent binocular rear vision in precisely the direction from which a hawk or a fox is most apt to approach. Something similar is true of the horse, who never has to turn his head to see behind him, as any Spanish rejoneador (bullfighter on horseback) can tell you. Each eye of a horse se
es 215° (probably a wider arc than any other known single eye), which gives him better than a 70° binocular field forward without any real blind spot behind.
The pupil in a horse's eye, incidentally, is horizontal, nicely attuning his day vision to the shape of his preferred landscape in the same way a cat's vertical pupil aligns itself to the natural feline habitat: a tree. The shape of other pupils can be fantastic. The gecko's looks like a string of four diamonds, the skate's a fan-shaped Venetian blind, the fire-bellied toad an opening like a piece of pie, the armored catfish a horseshoe, the penguin a star that tightens into a square, the green whip snake (whose ancestors may have slithered in the Garden of Eden) an appropriate keyhole, and others resemble teardrops, bullets, buns, crescent moons, hearts, hourglasses, boomerangs.
The angles of pupils have a sort of polarizing effect on the light entering the eye. This brings up the subject of polarized light, or light allowed to vibrate in only one plane. I mention it because polarized vision (awareness of light's polarity) is now known to prevail among most animals who navigate by sunlight and who have the obvious need to steer their course at a precise angle to the sun, whether or not it is hidden behind clouds, something only perception of the angle of polarity of sunlight could enable them to do. It is not generally known that man too can see polarized light and that the average human may see it without help from filters or other instruments. But this is demonstrably true. In fact anyone with normal eyes who stares upward for several minutes into clear twilight should gradually become aware of the shy and seemingly mystical retinal image known in optics as Haidinger's brush, a faint yellowish hourglass-shaped figure 4° long, squeezed at the waist between a pair of blue "clouds" and pointing exactly toward the sun.
Now going on to the very keenest natural vision among known creatures, we shall turn to the birds, particularly to such as hawks and kingfishers, who have evolved two foveas in each eye. The fovea is a special spot in the retina that's densely packed with cone-shaped cells (sensitive to daylight or its equivalent), where acuity is greatest, such as the image area of the two or three words on this page that you read with each little shift of your gaze. The reason hawks can see a mouse almost a mile away, or kingfishers a fish deep under the waves, is not that their eyes are bigger or more telescopic than yours or mine but because they have nearly eight times as many cells in their retinas, especially in the sensitive foveal areas which are packed with some 1,500,000 cone cells (as against your 200,000). The two foveas in each eye comprise a single lateral fovea for sharp monocular vision over a wide field to the side that's not seen by the other eye and a compound forward fovea for still sharper binocular vision straight ahead in the narrower stereoscopic field seen by both eyes. One eye or the other (not both) normally first notices the mouse or fish and, using its lateral fovea, concentrates on it during the downward swoop. Meanwhile the head turns and the other eye progressively converges forward until the compound fovea (compounded of both eyes) eventually takes over (binocularly) for closing in.
While daytime sight thus uses cone cells in the retina, night sight uses the 100,000 times more sensitive, long, rod-shaped receptors known as rod cells, which may respond to as little as one photon of light. These extraordinary cells are thought to have evolved in primordial fish during their prolonged struggles to go deeper, where it is safer and darker but where they could only see by gradually extending the detectable daylight with rod vision. Some 96 percent of all the kinds of fish who went down so deep that even the best rod cells could see no day, however, eventually evolved luminescence so they could at least see each other and recognize their fellow species by varied systems of colored lights and reflections, incidentally creating some fascinating problems in deception, camouflage, advertising, courtship and predation. But the other 4 percent of deep fish gave up vision altogether, including, surprisingly, a few species that evolved luminescence notwithstanding. Which would seem to confirm the discovery that it can be more important to be seen than to see, this being the reverse side of vision to which we will return presently.
Another aspect of vision is the functioning of the visual center in the brain, which learns to see through the actual visual experiences of the eye and retina. Evidence of this showed up in a classic experiment on kittens, in which they were allowed to see only vertical lines until they grew into cats. Then, exposed to horizontal lines for the first time, they could not see them at all and, since their fully grown brains had never developed pathways for horizontal vision, they were permanently horizontal blind!
You may recall that the eye, like other body parts, can be made out of any sort of flesh in the creature that grows it (page 153), but an eye can also be adaptable enough to migrate upon or through the body, even drift many times its own diameter on the initiative of its owner, without surgical or other outside help. I am thinking of nearly five hundred species of flat fish, most of which swim in a fish's normal vertical attitude when young with an eye looking out of each side of their heads. But fish of this order, in growing up, progressively take to resting on the sea bottom on one side, whereupon the down eye, finding itself blinded by mud and undoubtedly yearning for sight, begins a curious creeping migration up and around the ridge of the head (or, in some species, through it) until it arrives in a few weeks on the upper side, not only regaining but actually improving on its juvenile binocular conjunction. The mouth, incidentally, also twists Upward. And in some species of flounders, ichthyologists report, the fish has a choice as to whether to lie on its right or left side, and whichever eye it finally decides to turn downward is perforce the eye that will migrate to the up side (with equal facility) almost, one might say, with the freedom of an independent organism!
If you can swallow such an eye on a fish, what about an eye that can help a frog to swallow? Strange as it may seem, the West Indian tree frog has one. When he stuffs his mouth full of anything, which can include his own offspring, he retracts his bulbous eyeballs into his throat to push it down. Before doing so, he naturally has a good look around to be sure the coast is clear, because of course this way of swallowing temporarily blinds him, a loss which ironically has been known to induce him to throw up something delicious just for the sake of a quick gander at what might be, but usually isn't, going on.
If you have always thought of an eye as a positive, material organ, you may be interested to know about a negative, abstract eye that is an evolutionary success of long standing. I am thinking of the eye of the chambered nautilus, which consists of a retina and optic nerve in the back of a hollow socket that opens upon the outside oceanic world by means of a small vacuous pupil through which seawater flows. As this mollusk has no solid eyeball, no cornea, no iris and no lens, the pupil hole admits both water and the light needed to cast an image on the retina by the abstract principle of the pinhole camera, the ocean itself in effect serving as the nautilus's eyeball.
Then there is the arrowworm, a kind of microscopic swimming eye in the ocean, who, being transparent, sees with the help of his entire body, which refracts light and serves as a living, breathing cornea and lens. The core of his vision though is the combination of his two localized eyes, each sectioned into five sub-eyes, each of which has its own lens and retina and its own direction for looking (forward, backward, sideways, up and down), the down sub-eyes being the ones that most use the rest of the body to look through.
While it is hard to visualize this specialized five-way outlook of the arrowworm, a good deal is known of the more general and shallower view of the common and familiar fish of ponds and streams. For, knowing the reflectivity and the refraction index of water, we can deduce that the fish-eye view of this world is something like looking upward through a round hole in a horizontal mirror, the mirror being the underside of the water's surface (where air meets water) which reflects the bottom everywhere except directly overhead. Thus the sun, moon and stars, seen through that round hole, appear to rise and set in fish heaven at an angle of only 49° from the zenith (straight up) inste
ad of 90° as with us, their rays being bent as much as 41° on entering the water - so the fish can see the fisherman on the bank of a clear stream better and larger from the bottom than from near the surface. Indeed his porthole of upward sight is wider when looking
from there and it is said that the water's refraction has a magnifying effect while all around the hole he sees only underwater objects and their upper reflections.
OPTICAL ILLUSIONS
Fish probably share some of their illusions with humans, however, along with other creatures of vision. Consider the familiar bloated look of the rising or setting full moon, which affects any experienced eye viewing it from near the earth's surface. Although this remarkable illusion is known to have provoked theories throughout history, including one by Ptolemy (now accepted as true), Leonardo (off the mark) and others, most humans still seem puzzled by it. For this reason I am emboldened to explain that the true cause of the illusion is the natural capacity of the mind to compensate for the fact that all objects look smaller as you go away from them. Because you daily see familiar things like furniture, people, cars, birds, ships and airplanes diminishing as they go away or expanding as they come toward you, naturally the visual center of your brain makes allowance for your well-proven conviction that these things intrinsically stay the same size. So, in effect, they appear to you to maintain their constancy regardless of distance. This is true, among other things of course, of an object in the sky, like a balloon ten feet in diameter drifting a thousand feet above your head (subtending an angle of half a degree at your eye). For although such a balloon will diminish to a dot (of less than a hundredth of a degree) as it moves toward the distant horizon, you feel little doubt that its diameter remains ten feet. Yet if, by some miracle, the balloon, instead of diminishing to a dot, surprised you by maintaining its visual fullness (of half a degree) at the horizon, you would know it must have swollen enormously.
