THE TROPISM THEORY OF JACQUES LOEB 361 



In both cases we are entirely unable to analyze the phenomenon 

 if we wish to rest on a basis of fact. Nothing is more char- 

 acteristic of the tropism theory than the fact that it nourishes 

 best in these darkest corners of our knowledge. It is far easier 

 to construct a theory in regard to things of which one knows 

 practically nothing than about things which have been studied 

 thoroughly, for then the theory is in continual danger of being 

 impaled upon the rough edges of facts. These two tropisms 

 would be insufficient to impale the theory were it not for the 

 other arguments which are available as evidence in this case. 



THE IMPOSSIBILITY OF EXPLAINING THE PURPOSEFUL 

 NATURE OF TROPISMS 



After having considered the arguments against the tropism 

 theory from the morphological side, we shall now take up an 

 argument from the biological side, namely, the purposeful nature 

 of tropisms. It is usual to try to solve the problem of such 

 movements as are seen in tropisms, by considering the struc- 

 ture of the animal as a given premise, and by limiting one's self 

 to the movement as determined by certain elements of this 

 structure and by definite external stimuli. The purposeful 

 nature of the movement for the whole organism is then seen 

 as a consequence of the structure. Purposefulness is therefore, 

 wherever demonstrable, also a mere fact of observation, about 

 whose origin we need not trouble further. 



Loeb, however, takes an opposite view. For him, the struc- 

 ture of the organism is a negligible quantity which he quite over- 

 looks in reaching after the higher spheres of physical chemistry, 

 the universal panacea of modern investigation. The corner 

 stones of the tropism theory are merely the symmetry of the 

 animal and blind unorganized natural force, e.g., light. If from 

 the interaction of these two factors a truly purposeful action 

 results, it is most astonishing and some explanation is absolutely 

 demanded from Loeb. His only possible means of escape is 

 the selection theory, toward which, by the way, he takes a very 

 peculiar and extraordinary position. 



On one hand he denies any value to selection and writes: 

 " Whoever does not want to waste his time in an idle play of 

 words will do well to analyze instinct in the same manner as 

 is done for occurrences in the inorganic realm, where expres- 



