The Function of the Eye

Visual perception of direction in animals may show itself in quite another way, namely, in what is called phototaxis.

The following example illustrates what is meant by this. If a bee is taken, and released in the middle of a room, it flies with absolute certainty towards the window, and, therefore, towards the light. Such an animal is called positively phototactic. The cockroach illustrates an opposite condition ; it is negatively photo tactic. It avoids the light, and when turned out of its hiding place, it will assuredly make for the dark.

That phototaxis is not always a constant character is shown by the bee. We cannot say off-hand "the bee is positively photo tactic." It is so only under quite definite conditions; in very many other circumstances it is not so. For example, when it returns from gathering honey, and flies into the dark hive, it is much rather to be described as negatively phototactic.

Sometimes it is easy to state the conditions under which an animal will become phototactic. The bee becomes positively phototactic if it is caught, and strives to free itself. In nature, where there are no glazed windows, brightness means the open air and the sky. A very clear example of phototaxis appearing only under certain conditions is furnished by the water scorpion (Nepa cinerea). This creature lives in shallow waters among the plants on the bottom. It is, however, an air-breather, and is obliged to ascend to the surface of the water from time to time, in order to inhale it. In this it is aided by positive phototaxis, which, in this case, co-operates with negative geotaxis. Since light only enters the water from above, the creature, which becomes phototactic only with lack of air, creeps up the water plants as much as possible in a vertical direction, until it reaches the surface. Scarcely, however, has it breathed in the fresh air, when phototaxis ceases, and the water scorpion is able to descend again into the deeper regions. As a final example, the water-flea, Daphnia, may be mentioned, in which we can produce phototaxis at will. We have only to introduce water containing carbon dioxide into a vessel in which there are Daphnia, to see at once that all the animals swim as quickly as possible towards the light.

Here, however, we must again make it clear that, normally, the light enters from above. Daphnia live in stagnant waters, fre quently very unclean, at the bottom of which poisonous carbon dioxide can be very easily formed through putrefactive processes. In this instance, phototaxis serves to protect the Daphnia from poisoning by carbon dioxide.

The Mechanism of Phototaxis.

While these matters are comparatively simple, the naturalists of the present day are as yet very divided in their opinions regarding the mechanism of phototaxis. There are various theories, of which we may, per haps, go so far as to say that each is correct in some instances. The best known is the Theory of Tropisms (also called the Ray-Verworn Theory, or the Theory of Tropotaxis). It is founded on the fact that the majority of animals which are prototactic have bilateral symmetry. These all have a median plane dividing the body into two, absolutely identical halves. The entire animal is regarded as a small automaton. The light rays which fall on the eye are converted into nervous energy, which is conducted to the brain. The latter transmits the stimulus to the limbs, which are under its control, and thus, according to this theory, the movements of the legs are, by a natural law, depend ant on the strength of the illumination of the eyes. If the rays of light come from the front, they strike both eyes equally, and, therefore, the legs of each side move with equal force, and the animal runs straight forward. If, however, the light comes from one side, so that one eye is more strongly illuminated than the other, there is also a difference between the two sides in the working of the legs. The conse quence must be that the animal turns until its median plane coin cides with the direction of the light rays. When this is accom plished, it again runs in a straight line towards the light. This theory is so very striking in its great simplicity that, at first, it was received with much enthusiasm, particularly by everyone inter ested in Natural Philosophy.