THE TROPISM THEORY OF JACQUES LOEB 347 



CASES IN WHICH THE PREMISED CONDITIONS OF SYMMETRY AND 



OF ENERGY RADIATION ARE PRESENT, BUT THE MOVEMENT 



TAKES PLACE IN A WAY THAT IS CONTRARY TO 



THE TROPISM THEORY 



Sidewise movement of crabs. — Three examples only will be 

 cited of tropisms that take place in a manner that is contrary 

 to the theory. The clearest of these, already discussed by 

 H. S. Jennings, is the sidewise movement of crabs. The facts 

 may be stated in a few words: If a crab is stimulated in any 

 way, for example, by a strong light and from one side only, 

 there is no turning of the animal until its axis of symmetry 

 coincides with the ray of light, as the tropism theory requires, 

 but at once the crab moves sideways away from the light. This 

 cannot be explained at all by Loeb's theory; it shows, rather, 

 in the clearest way that the light does not act separately upon 

 the two body halves, but upon the body as a whole, and, in 

 consequence, a harmonious activity of all organs of movement 

 is the result. Loeb, indeed, makes some objections to this 

 criticism. He writes: ' I am rather inclined to draw another 

 conclusion, namely, that in the first place, in crabs an entirely 

 different relation exists 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 superficial elements. There is here, in my opinion, 

 a new discovery to be made." (1909, pp. 48-49.) 



It will be seen later why Loeb suddenly calls to his aid the 

 special structure that he has hitherto neglected, but it would 

 be difficult to say that his reply has in any way weakened the 

 evidence against the theory. In this case an error has certainly 

 crept into Loeb's reasoning. His explanation means, in some- 

 what plainer words, that in the crab in supposed opposition to 

 other Crustacea the eye is not only related with the legs of one 

 side, which causes a turning of the animal, but with the legs 

 of both sides, which results in another kind of movement. The 

 real meaning of the tropism theory is not, however, that light 

 may cause any desired movement, but that it does cause a 

 definite movement, namely, that which leads the animal away 

 from the light; and, as Loeb himself has emphasized again and 

 again, this is only possible, in accordance with his theory, when 



