38 The Mechanistic Conception of Life 



direction of their progressive movement is just as unequivocally 

 governed by the source of light as the direction of the move- 

 ment of the planets is determined by the force of gravity. 



The theory of the compulsory movements of aphids under 

 the influence of light is as follows : Two factors govern the pro- 

 gressive movements 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 photochemical action of light, 

 we know today that a great many chemical reactions of organic 

 bodies are accelerated by light. Especially is this true of oxi- 

 dations.^ The mass of facts is already so great that we are 

 justified in assuming 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, e.g., oxidation, increases. 



The second factor is the symmetrical structure of the ani- 

 mal. As expressed in the gross anatomy of the animal, 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, by which I mean that symmetrical 

 regions of the body are chemically identical and have the same 

 metabolism, 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 retinae, 

 which are certainly symmetrical, have an identical metabolism, 

 while a region of the skin which is not symmetrical with the 

 retina has a different metabolism. The individual points on 



1 Luther, Die Aufgaben der Photochemie, Leipzig, 1905; C. Neuberg, Biochem. 

 Zeitschr., XIII, 305, 1908; Loeb, The Dynamics of Living Matter, New York, 1906. 

 In addition, see the work of Ciamician, as also of Wolfgang Ostwald (Biochem. 

 Zeitschr., 1907). 



