THE SIGNIFICANCE OF TROPISMS 113 



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 sub- 

 stance. Now every animal is continually producing acids in its cells, 

 especially carbonic acid and lactic acid. It probably produces also sub- 

 stances 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 ac- 

 tive mass of the photosensitive substance in a copepod is relatively 

 small, a temporary increase in the production of carbonic acid can in- 

 crease 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 move- 

 ments 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 physiolog- 

 ically, 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 explana- 

 tions 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 suffi- 

 ciently photo-sensitive have caused many writers to assert that helio- 

 tropic animals do not place themselves directly in the line of the rays 

 of light, 9 but that they first have to learn the right orientation. But 

 a very striking experiment contradicts this assertion. The larva? of 

 Balanus perforatum develop entirely in the dark. If the ovary filled 

 with mature larvae is, in the dark, placed in a watch crystal filled with 

 sea water, the larvae emerge at once and, if they are brought into the 



•Provided that only a single source of light is present. 



