io8 THE POPULAR SCIENCE MONTHLY 



ments of the animals under these conditions; one is the symmetrical 

 structure of the animal, and the second is the photochemical action of 

 light. "We will consider the two separately. In regard to the photo- 

 chemical action of light, we know to-day that a great many chemical 

 reactions of organic bodies are accelerated by light. Especially is this 

 true of oxidations. 4 The mass of facts presented to us here concerning 

 this relatively young branch of physical chemistry is already so great 

 that we have the right to assume that the determining action of light 

 upon animals and plants is in its last analysis due to the fact that the 

 rate of certain chemical reactions in the cells of the retina or of other 

 photosensitive regions of the organisms is modified by light; with 

 increasing intensity of light the rate of certain chemical reactions, for 

 instance, oxidation, increases according to a definite law, namely, the 

 law of Bunsen and Eoscoe. 



The second factor is the symmetrical structure of the animal. As 

 expressed in the gross anatomy of the animal this is shown by the well- 

 known fact that the right and left halves of the body are symmetrical. 

 But it is my belief that such a symmetry exists in a chemical sense as 

 well as in an anatomical sense — by which I mean that symmetrical 

 regions of the body are chemically identical and have the same metab- 

 olism, while non-symmetrical regions of the body are chemically 

 different, and in general have a quantitatively or qualitatively different 

 metabolism. In order to illustrate this difference it is only necessary 

 to point out that the two retinas, which are certainly symmetrical, have 

 an identical metabolism, while a region of the skin which is not sym- 

 metrical with the retina has a different metabolism. The individual 

 points on the retina are also chemically unlike. The observations upon 

 visual purple, the differences in the color-sensitiveness of the fovea cen- 

 tralis, and the peripheral parts of the retina indicate that the points of 

 symmetry of the two retinas are chemically like, but the non- 

 symmetrical points chemically unlike. 



Now if an. unequal amount of light falls upon the two retinas, 

 the photo-chemical reactions in the retina which receives more light 

 will also be more accelerated than in the other. The same thing nat- 

 urally holds true for every other pair of symmetrical photosensitive sur- 

 face elements. For it should be mentioned just here that photochemical 

 substances are not only found in the eyes, but also in other places on the 

 outer surface of many animals. In planarians, as my experiments and 

 those of Parker have shown, not only the eyes, but also other places on 

 the skin, are photosensitive. But if more light falls upon one retina 

 than upon the other, the chemical reactions, for instance, the or- 



4 Luther, "Die Aufgaben der Photochemie, " Leipzig, 1905. C. Neuberg, 

 BiocJiem. Zeitschr., Bd. 13, S. 305, 1908. Loeb, "Vorlesungen iiber die Dynamik 

 der Lebenserscheinungen, " Leipzig, 1906. In addition, the work of Ciamician, 

 as also Wolfgang Ostwald (Biochem. Zeitschr., 1907). 



