4. The facetted eyes of insects, in which a stippled image is formed, on the principle of mosaic vision.

Unfortunately, all these are called indiscriminately eyes, or organs of vision. An infusorian or a snail is said to see. But the terms "eye," "vision," "sight," imply that final excellence to which only the higher animals, each on its own line, have attained.

This final excellence probably has its basis and earliest inception in the fact that the functional activity of protoplasm is heightened in the presence of ætherial vibrations. If, then, we imagine, as a starting-point, a primitive transparent organism with a general susceptibility to the influence of light-vibrations, the formation within its tissues of pigment-granules absorbent of light will render the spots where they occur specially sensitive to the ætherial vibrations. Special refraction-globules would also act as minute lenses, focussing the light, and thus concentrating it upon certain spots.

Fig. 39.—Direction-retina.

Simple retina for distinguishing the direction of the source of light or of shadow.

In many of the lower animals we find such organs, belonging to our first category, and constituting either eye-spots of pigmented material or simple lenses covering a pigmented area. If we call these eyes, we must remember that in all probability they have no power of what we call vision—only a power of distinguishing light from dark. Where, however, there exists beneath the lens a so-called retina, that is, a layer of rod-like endings of a nerve, it might, at first sight, be thought that there, at any rate, we have true vision. But in all probability, in a great number of cases the retinal rods are simply for the purpose of rendering the organism sensitive, not only to the presence of light, but to its direction. Light straight ahead (a) stimulates the middle rods; from one side (b, c) it is focussed on the rods of the opposite side of the retina; and similarly for intermediate positions. The presence of a retinal layer is thus no infallible sign of a power of vision as apart from mere sensibility to light. Indeed, in a great number of cases, from the convexity and position of the lens, the formation of an image is impossible. Only when it can be shown that a more or less definite image can be focussed on the retina, or can be formed on the principle of mosaic vision, can we justly surmise that a power of true vision is present. I doubt whether this can be shown to be unquestionably the case in any forms but the higher arthropods, the cuttle-fishes and their allies, and the vertebrates.

There is one more point for consideration before we leave the sense of sight—Are the limits of vision the same in the lower forms of life as they are in man? or, to put the question in a more satisfactory form—Are the limits of sensibility to light-vibrations the same in them as in us? M. Paul Bert concluded that they are. But Sir John Lubbock has, I think, conclusively shown that they are not. For the full evidence the reader is referred to his "Senses of Animals."[FM] His experiments on ants, with which those of M. Forel are in complete accordance, satisfied him that these little animals are sensitive to the ultra-violet rays which lie beyond the range of our vision. Other experiments with fresh-water fleas (Daphnia) showed that they have colour-preferences, green and yellow being the favourite colours.

The daphnias were placed in a shallow wooden trough, divided by movable partitions of glass into divisions. Over this was thrown a spectrum of rainbow colours. The partitions were removed, and the daphnias allowed to collect in the differently illuminated parts of the trough. The partitions were then inserted, and the number of crustaceans in each division counted. The following numbers resulted from five such experiments:—

Dark.Violet.Blue.Green.Yellow.Red.
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