§ 2. Differentiation of Sense Organs.—Degree of discriminative sensibility corresponds broadly to the complexity and differentiation of the organs of sense. If the nerve fibres running to the skin in human beings are laid bare and directly stimulated, "then, however they be stimulated, be the stimulus weak or strong, if consciousness be affected at all, the affection takes on the form of pain; psychological examination of the subjective result discloses nothing that can be called a sensation of touch."* Touch or pressure-sensations, delicately differentiated as they are, and almost neutral in tone, and capable of combining in one moment of consciousness a great variety of qualitative differences, can only be developed by the help of special terminal organs. But cutaneous pain-sensations, and all organic sensations which are vague, diffusive, and stronglytoned, arise without the help of specially differentiated endorgans. Now, in the ascending scale of animal life, we find a growing complexity and differentiation of the terminal organs of sense and of their nervous connexions, marking a correspondingly graduated displacement of sensational by perceptual consciousness.

In following the ascending scale of animal life, we find a gradual evolution of specialised structures for the reception of special kinds of external stimulation; beginning with those which are scarcely distinguishable from the general surface of the body, and ending with such elaborate organs as the human eye or ear. The best illustration is drawn from sight, because most is known about it. It must be understood that the word "sight" is here used to mean merely "sensitiveness to light." It must not be assumed that the sensations produced by luminous vibrations are the same in the higher organisms as in the lower.

*Foster, TextBook of Physiology, p. 1427.

In some lowly organisms which have no eyes the general surface of the body appears to be sensitive to light. This is the case with earthworms and newts. "It is easy to imagine," says Lubbock, "that in unpigmented animals whose skins are more or less semitransparent, the light might act directly on the nervous system even though it could not produce anything which could be called vision."* Certainly it would be misleading to call the experience of the earthworm a visual sensation. We must rather suppose it to be a kind of general organic discomfort.

The most rudimentary beginning of a special structure for the reception of lightstimulation consists simply in groups of pigmented cells with a nervous connexion. The pigmented material occurring in a semitransparent organism arrests and absorbs the light. The limpet has eyespots of this simple kind "on the outer side of the tentacles where the eyes are situated in more highly organised species, "+ The skin is thrown into a pit within which the epithelial cells are elongated and pigmented.

The next step is the development of a lens for condensing the light in the manner of a burningglass. Some species of worms have only pigmented cells, others have a concentrating apparatus. These simple eyespots, consisting of pigmented cells and a vitreous body or condensing lens, may exist in great numbers over the general surface of the organism. Thus in a species of worm called "Polyophthalmians "there is a series of eyespots "along the sides of the body, in pairs from the seventh to the eighteenth segments."+ Such rudimentary organs can only serve to render the creature sensitive to degree of illumination, to the transition from light to darkness; they thus make possible a protective reaction when the shadow of an approaching object falls on the animal.

* The Senses of Animals, p. 207.          + Lubbock, op. cit., p. 139.

+ Op, cit., p. 134.

The next important step is the development of a rudimentary retina, essentially consisting in a layer of rodlike nerveendings. The eye of the snail is situated on its hinder horn or tentacle. It consists of a cornea or transparent horny integument, a lens, and a retina composed of three layers, (1) the rods, which are the proper organ of vision, (2) a cellular layer, (3) a fibrous layer. "In all probability the eye does little more than enable the snail to distinguish between light and dark." "It does not seem to be aware of an object unless it is brought within a quarterofaninch of its tentacle."* The rods of the retina in which the optic nerve terminates in all probability merely render the animal differentially sensitive to different directions of the light. In many animals which possess these retinal rods the formation of an image in any way comparable to that thrown on the retina of the human eye is impossible from the position and convexity of the lens. These eyes with rudimentary retinas, more or less sensitive to direction, may be spread in great numbers over the surface of the body. There are certain species of a genus of seashore slugs called Onchidium which have these scattered eyespots in varying numbers, some a hundred, others as few as twelve. The number differs in different individuals of the same species, and the eyes "are continually growing and being reabsorbed."+ The back of the Onchidium contains a number of glands, each opening by a minute pore; and it has been suggested that when warned by the shadow of certain flyingfish which come out of the sea to prey upon them, the little slugs emit a shower of spray and so drive off their enemy.

* Lloyd Morgan, Animal Life and Intelligence, p. 293. + Lubbock, The Senses of Animals, p. 143.

The next stage in the development of the eye is the formation of a retinal image by means of a lens; it is necessary for this that each diverging pencil of rays from a point in the object shall be brought again to a focus in one point, and in only one point, of the retina. The delicacy and perfection with which this is effected depends on the complexity of structure of the retina, on the nature of the lens, and on the power of adjusting it for different distances. Cuttlefish and their allies have welldeveloped apparatus for the formation of images. So have vertebrate animals, but of course in varying degrees. Many fishes do not distinguish their food (worms) at a greater distance than three or four feet. On the other hand, some of them have very accurate vision for short distances. "I have often seen," says Mr. Bateson, "a large Wrasse search the sand for shrimps, turning sideways, and looking with either eye independently, like a chameleon. Its view is so good that it can see a shrimp with certainty when the whole body is buried in grey sand, excepting the antennae and antennae plates."* Some reptiles and amphibians have similar accuracy of vision at short distances.

* Quoted by Lloyd Morgan, Animal Life and Intelligence, p. 287.

Besides this main line of development of the visual organ which leads up to the eye of vertebrates, with its apparatus for forming a distinct image by means of a lens and delicately sensitive retina, there is a branch line which leads to the compound or facetted eye of insects and of Crustacea such as crabs and lobsters. The surface of these compound eyes is divided up into a great number of hexagonal areas, each of which is called a facet, and in some insects forms a little lens. A kind of dragonfly is stated to have twenty thousand of these hexagonal facets. Beneath each facet is a crystalline cone, with its base towards the facet and its apex turned inwards, where it ends in great elongated cells; in the midst of these there is a nerverod. Dark pigment is developed round each of the cones. "Starting from a simple form of eye consisting of a lens and a nervefibre, we should arrive at the compound eye by bringing together a number of such eyespots, and increasing the number of lenses, while the separate cells beneath each lens coalesced to form a single crystalline cone and rod." As regards the way in which these eyes perform their function, there has been much dispute. But it is now pretty clearly made out that the facetted organs taken collectively fulfil in a different way the same office as the lens in the eye of vertebrates. Only those rays of light which go straight through a crystalline cone affect the nerverod. All the rest which strike the cones obliquely are absorbed by pigment. Thus, each of the cones conveys to its own nerverod a single minute spot of light coming from a single point in the field of view, and from that point only. The result is what Lloyd Morgan calls a "stippled image."* The range of vision with such eyes is much smaller, and the image which they form must be far less accurate and distinct than in the higher vertebrates.

* Op. cit., p. 290,