UR human world is a very limited part of nature. The unaided senses of primitive man open a few doors of communication between the individual and his surroundings, through which the sum total of his knowledge of things as they are must be derived. Science has greatly enlarged the efficiency of the natural sense organs-the microscope and the telescope have extended the range of vision, the periscope enables us to see around a corner, the spectroscope, photographic plate, X-ray machine, and innumerable other aids have enabled us to see deeper into nature. But no new senses have been developed and our furthest scientific advances and most recondite philosophical theories must be based in last analysis on such fragmentary knowledge of the cosmos as is revealed to us by our senses. Great realms of nature remain wholly unexplored, although new artificial aids permit constant advances into the hitherto unknown-Hertzian waves and wireless telegraphy, ions and the new chemistry, electrons and the new physics.

Fortunately the traditional five senses do not represent our whole physiological equipment for this task. In fact, the human animal is endowed with about twenty distinct senses, including two in the ear, at least four in the skin, and numerous others in the deep tissues such as muscle sense, hunger, thirst, and other visceral senses.

It is well known that fishes and other lower vertebrates possess numerous types of sense organs quite unlike anything in our own bodies, and it is quite impossible for us to form any conception of what the world appears like to these animals except in so far as their sensory equipment is similar to our own. Even the companionable dog, who responds so sympathetically and intelligently to our moods, lives in a very dif

ferent world. Recent experiments have shown that his sense of vision is very imperfect, especially for details of form, and everybody knows the inconceivable delicacy of the hound's sense of smell. With us vision is the dominant sense and our mental imagery is largely in terms of things seen. Even a blind man will say, "I see how it is," when he comprehends a demonstration.

What sort of a world is it to a dog, whose finest experiences and chief interests are in terms of odors? And how does it feel to be a catfish, provided not only with large olfactory organs whose central nervous centers make up almost all of the cerebral hemispheres of the brain but also with innumerable taste buds all over the mucous lining of the mouth and gills and freely distributed over the entire outer skin from the barblets ("feelers") around the mouth. to the tail fin? We cannot conceive the epicurean delights which such an animal may feel when he swims into the water surrounding a juicy piece of fresh meat, by whose odorous and savory juices he is bathed. One wonders, parenthetically, how far the fish himself is able to conceive or even enjoy the pleasures of life. With how much mind of any sort the fish is endowed is at present an unsolved riddle; but it is certain that his behavior complex is of very different pattern from ours and whatever mind he may have would surely be as different as the pattern of his sense experience is different.

Let us pursue this line of inquiry further and review what is known of the other senses of our catfish. This fish has small and poorly developed eyes and is largely nocturnal in habit, lying concealed in dark corners during the day. The retina has remarkable powers of adaptation to differences in illumination and the fish is very sensitive to changes in intensity of light. But the

eve is not the only light-sensitive organ. Experiments with blinded fish show that the entire skin surface is sensitive to differences of light intensity, a not uncommon feature of aquatic animals. The image-forming power of the eye is probably not very good. Some catfishes, it is true, will take a spoon hook, and probably a bait must always be in motion if it is to be sensed by the eye. The usual method of feeding is to trail the bottom with the barblets, which are very efficient organs of both touch and taste, and when contact is made with a worm or other suitable food to turn sharply and snap it up.

Just as the eyes are supplemented in their functions by the skin, which has a very feeble sensitiveness to light, so the highly refined chemical sense organs in the nose and taste buds are also supplemented by a chemical sense in the general skin. In some other fishes which have been carefully tested the general skin surface is found to be very sensitive to chemicals in solution, to some substances more sensitive, in fact, than are the taste buds themselves.

In fishes, as in men, the ear contains two quite different sense organs-the organ of hearing and the organ of the sense of equilibrium. The latter lies in the semicircular canals, which in form and function are similar to those in the human body. Indeed, the semicircular canals probably play a larger part in the behavior of the fish, since maintaining perfect equilibrium is a more difficult matter for a fish suspended in water of about the same specific gravity as the body than for a man walking on solid ground. But when the man essays to fly, his semicircular canals again take a dominant place in his sensory equipment. In the practical testing of the fitness of men who are candidates for the Air Service of the Army the most important point to be determined. is whether the semicircular canals are functioning normally.

Whether fishes hear at all has been

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hotly controverted. That they are very sensitive to mechanical jars and vibrations all agree, but it has been difficult to prove whether their responses to these vibrations are brought about through their ears or by refined cutaneous sensibility. The ingenious experiments of Parker1 have shown that both of these organs serve and that, in fact, fishes do hear true sound waves of rather low pitch with their ears. То tones of high pitch they are deaf and probably they have no power of tone analysis, that is, they can hear a noise but cannot tell one tone from another.

The fishes can boast no superiority over ourselves in being able to respond to low tones by both the ear and the skin. We can do the same, as can readily be shown by lightly touching the sounding board of a piano or organ when a low tone is struck. The same tone heard by the ear can be readily felt by the finger tips. But for perceiving still slower vibratory movements we, with all our boasted brain power, must admit ourselves inferior to the fishes. They possess an elaborate system of cutaneous and subcutaneous sense organs of which we have not a vestige. These so-called lateral line organs in the catfish comprise a complex system of fine tubes under the skin, the lateral line canals, and two kinds of sense organs in the skin, the pit organs. The canals ramify in various directions in the head and the main lateral canal extends along the side of the body back to the tail. They were formerly supposed to be for the secretion of mucus and are still often called the mucous canals. But they are now known to contain numerous small sense organs which respond to slow vibratory movements of the water. The pit organs are scattered over the skin, the smaller ones each in a flask-shaped pit with a narrow mouth and the less numerous larger ones exposed on the surface.

1 See recent work on this subject by Prof. G. H. Parker, of Harvard University.