THE TROPISM THEORY OF JACQUES LOEB 365 



tent everybody, for it requires the acceptance of certain un- 

 proved assumptions, but at least it is not contrary to facts. 



It seems to me of the utmost importance that tropisms should 

 not be taken out of the class of other reflexes. If we observe 

 the reactions of the animal to light, from the simplest sensory 

 reflex up to image perception there is a continuous chain of 

 increasingly complex reflexes, whose common attribute is that 

 with the light stimulus definite muscles become active. Phy- 

 siologically, therefore, these movements are all alike; it is con- 

 sequently inadmissable to explain some of them in a way that 

 contradicts what is true of all the others. 



It is hoped that the arguments here brought together will be 

 sufficient to make the tropism theory disappear from zoological 

 literature. 



The theory is useful only in so far as it shows that the appli- 

 cation of the inorganic sciences to the problems of biology is 

 of very limited range. I do not wish to be misunderstood upon 

 this point. There are certain problems, and many of them of 

 great importance, which can only be solved with the aid of 

 chemistry and physics. But such problems always concern 

 merely the study of single organs, perhaps of muscle, which is 

 evidently a chemical energy-machine, or of the alimentary 

 tract, which represents a chemical manufactory. Here and in 

 a thousand analogous cases, with which naturally the physiology 

 of the senses belongs, the physical chemist finds a rich field of 

 activity. Just as soon, however, as we observe many organs 

 working together, whether in a system of organs or in a whole 

 organism, we are confronted everywhere, in the morphological 

 structure as well as in all physiological processes, or the so- 

 called " psychic " actions of the animal, by adaptations of pur- 

 poseful nature'. Whoever denies this, only shows that he is 

 insufficiently acquainted with facts. Purposeful structures, 

 however, cannot be explained by physical chemistry. The 

 chief fault of Loeb and his followers is that they have overlooked 

 this point. 



We have found it universally true that in all tropisms the 

 fundamental mechanism of movement is adaptively constructed, 

 for the purpose of leading the animal toward or away from the 

 source of energy. Loeb's attempt to refer the actions of the 

 lower animals to chemo -physical processes should therefore be 



