THE TROPISM THEORY OF JACQUES LOEB 351 



as an example. As soon as this animal is removed from its 

 normal horizontal position, with ventral side up, involuntary 

 movements restore the original position. This kind of orient- 

 ing movement can be reconciled with the tropism theory only 

 when a turning around the long axis occurs. But place the 

 Palaemon nearly perpendicular to the surface, and it will recover 

 its normal position by turning around the horizontal cross axis, 

 which, as we saw above, prevents any explanation by Loeb's 

 theory. 11 



These diatropic movements cannot be ignored. They rep- 

 resent typical movements of orientation with reference to a 

 source of energy, and are thoroughly genuine tropisms. Loeb 

 must accustom himself to the idea that, in addition to the side- 

 wise movements of crabs, there is another great category of 

 phenomena which do not agree with his theory. 



Without dwelling longer on the examples of diatropic phe- 

 nomena, which after all, are relatively infrequent, let us con- 

 sider any organism with positive or negative heliotropism which 

 moves freely in space, perhaps swims. When the source of 

 light is really on one side of it, the animal will conform entirely 

 with the scheme of the theory by turning around a vertical axis 

 which lies in the plane of symmetry, but if the light is either 

 above or below it, then the animal will turn, as any experiment 

 will show, around the cross axis, which is, as we know, con- 

 tradictory to the theoretical expectations. 



We see here once more what the observation of Squilla and 

 Palaemon has already shown us, that in one and the same ani- 

 mal certain tropisms 'occur which seem to be explained by the 

 theory, together with others which are seen at the first glance 

 to be entirely inexplicable. Surely no one would wish to sepa- 

 rate these two movements, the turning around the vertical, and 

 the turning around the horizontal axis, or to assert that although 

 the first is conditioned by the general laws of the tropism theory, 

 the second requires a very different kind of explanation, namely 

 that of the specific structure of the animal. No! Both move- 

 ments, which frequently replace each other and can be com- 

 bined in various ways, are fundamentally alike. Since it can 



11 Palaemon reacts also after loss of the statocysts, as I have shown, by means 

 of the general posit ion -reflex, which, since we know little about it in detail, may 

 itself be open to explanation by the tropism theory. 



