THE TllOPISM THEORY OF JACQUES LOEB 343 



only in an anatomical sense, but in a chemical sense; by which 

 I mean that symmetrical body regions are chemically identical and 

 have the same reactions, while asymmetrical body regions are 

 chemically different, and in general have quantitatively or quali- 

 tatively different reactions. * * * If now more light falls on 

 one retina than on the other, the chemical reactions also, for 

 example the organic oxidations, will be more accelerated in one 

 retina than in the other, and correspondingly greater chemical 

 changes will occur in one optic nerve than in the other. 



This inequality of the chemical processes spreads from the 

 sensory to the motor nerves and finally to the muscles with 

 which the latter are connected. We conclude from this that 

 with an equal illumination of both retinas the symmetrical 

 groups of muscles of both body halves are influenced chemically 

 in the same way and so are in the same state of contraction; 

 while, if the reaction speed is unequal, the symmetrical muscles 

 on one side of the body are in greater activity than on the other 

 side. The result of such unequal activity of the symmetrical 

 muscles of both body halves is a change in the direction of 

 the movement of the animal. 



This alteration of the direction of the movement can either 

 cause the head to turn toward the source of light, so that con- 

 sequently the whole animal moves in the direction of the source 

 of light; or else to turn the head in the opposite way and the 

 animal moves in the opposite direction. * * * As soon as this 

 has happened the two retinas are equally lighted and the (sym- 

 metrical) 3 muscles in both body halves once more work with 

 the same strength. In consequence there is no longer any 

 reason that the animal should deviate from this direction to 

 one side or the other. It is therefore automatically led to the 

 source of light. The will of the animal, which in this case 

 dictates his direction, is the light, as it is gravity in the falling 

 of a stone or the movement of the planets." (1909, pp. 9-14. ) 4 



Jennings has aptly termed this theory, the "theory of local 

 actions," because it does not regard tropism as a movement of 

 the animal as a whole, but as a movement of both body halves, 

 which work against each other. This we must remember as an 



3 Italics and (symmetrical) added by the present author. 



4 Loeb, Jacques. Die Bedeutung der Tropismen fur die Psychologie. Vortrag 

 gehalten auf dem VI. Internationalen Psychologenkongress zu Genf. 1909. 



