toward the source of light only under strong illumination, but that under weaker illumination an essentially different condition exists.
Still another point is to be considered. We have seen that acid increases the sensitiveness of certain animals to light and probably, as we assume, by increasing the active mass of the photochemical substance. Now every animal is continually producing acids in its cells, especially carbonic acid and lactic acid. It probably produces also substances which could have the opposite effect and which decrease the heliotropic sensitiveness of the animals. Fluctuations in the rate of production of these substances will also produce fluctuations in the heliotropic sensitiveness of the animal. Now if, for instance, the active mass of the photosensitive substance in a copepod is relatively small, a temporary increase in the production of carbonic acid can increase the photosensitiveness of the animal sufficiently for it to move for the period of a few seconds directly toward the source of light. Later the production of carbonic acid decreases and the animal again becomes indifferent to light and can move in any other direction. Then the production of carbonic acid increases again and the animal goes again, for a short time, toward the light. Such animals finally gather at the lighted side of the vessel because the algebraic sum of the movements in the other directions becomes zero according to the law of chance. But it is plain that such animals do not reach the source of light by a straight path. A writer who is not trained to interpret the variations in the behavior of such an animal chemically and physiologically, can naturally give no explanation of their significance. If he is forced to find an explanation he will wind up at the method of "trial and error" which is no more chemical nor scientific than the explanations of metaphysicians in general.
Some authors have, it seems, worked only with animals which were not pronouncedly heliotropic and the photo-sensitiveness of which wavered about the threshold of stimulation in the manner described above. A writer trained in physical chemistry would have understood that such animals are unsuitable for experiments in heliotropism and that it is necessary to first increase their photo-sensitiveness if the laws of the action of light upon them are to be investigated.
I also believe that observations upon animals which are not sufficiently photo-sensitive have caused many writers to assert that heliotropic animals do not place themselves directly in the line of the rays of light, but that they first have to learn the right orientation. But a very striking experiment contradicts this assertion. The larvæ of Balanus perforatuS develop entirely in the dark. If the ovary filled with mature larvæ is, in the dark, placed in a watch crystal filled with sea water, the larvæ emerge at once and, if they are brought into the
- Provided that only a single source of light is present.