THE TROPISM THEORY OF JACQUES LOEB 357 



between the retina and the locomotor muscles than in other 

 Crustacea and in most animals; and that in the second place 

 an especial peculiarity exists in regard to the function of the 

 two retinas, in that these do not behave as symmetrical super- 

 ficial elements." 



He therefore not only does not deny the existence of the 

 reflex arc, but uses it to explain the existence of the phenomenon, 

 and yet he still stands by the tropism theory in toto. The solu- 

 tion for this enigma has escaped his recent critics ; it will only be 

 discovered by recalling the historical development of the tropism 

 theory which may be told as follows: The phenomenon which 

 shows the peculiarities of tropic movements in the clearest and 

 sharpest manner is undoubtedly galvanotropism; the theoretical 

 estimate of the remaining tropisms was then made by analogies 

 drawn from this first phenomenon. Now the relations in gal- 

 vanotropism are of a very special character. It does not occur 

 in nature and is a pure laboratory product. So far as it is 

 concerned, the animal is no machine with the definite tasks 

 of the maintenance of its own life and the propagation of the 

 species, but is solely a symmetrical form the two sides of which 

 possess a similar chemo- physical structure. Expressions like adap- 

 tation and function are not needed here, and if a future chem- 

 ist should ever succeed in producing from inorganic matter 

 an animal like Faust's homunculus, this being would have no 

 relation to function, natural selection, adaptation, etc., but 

 would infallibly manifest the phenomena of galvanotropism. 

 The organism in this case might be compared with a crystal, 

 which does in fact possess a structure, from which on the appli- 

 cation of certain energies definite resultant phenomena are mani- 

 fested, but to which the conception of adaptation would be 

 wholly inapplicable. 



The fundamental mistake which Loeb and his followers make 

 is to apply indiscriminately what they find true for galvanotro- 

 pism to all tropisms that occur in nature. The idea of adap- 

 tation is here no empty delusion. If Leob actually denied the 

 statement that the animal is a machine adapted for self-main- 

 tenance and for the propagation of the species, then no further 

 discussion would be possible, but for his own sake, and for the 

 sake of his reputation as an investigator of nature, I assume 

 that he does not g6 so far. If, however, he is prepared to admit 



