THE TROPISM THEORY OF JACQUES LOEB 363 



Finally, there are many cases in which positive heliotropism 

 forces great masses of young larvae to make purposeful move- 

 ments which scatter them abroad in the water. There are many 

 more examples and they will certainly increase in the future 

 as our knowledge of the mode of life of the lower animals be- 

 comes more profound. 



How can Loeb, if he stands by the tropism theory, explain 

 the evidently purposeful nature of these tropisms? 



The exact course of action in such movements must be con- 

 sidered if one is really to understand them. In the case of the 

 Porthesia caterpillar hunger is the conditio sine qua non for the 

 occurrence of the tropism, and this, in its turn, leads the cater- 

 pillar to the nourishing leaves. We have here a physiologically 

 harmful condition, which of necessity causes a movement which 

 ends the harm. Hunger is therefore to some extent its own 

 physician ! 



Few words are required to show that this is evidently not a 

 .simple but a very complicated occurrence. If we wish to explain 

 its origin with the help of the selection theory then it must be 

 assumed that an elimination of unfit individuals takes place in 

 two ways. On the one hand, all those caterpillars must perish 

 which do not • move directly toward the light but which move 

 in a diaheliotropic manner, and of the selected individuals all 

 must die which manifest the tropism under other circumstances 

 than those of hunger. For should the impulse remain after the 

 twigs are stripped bare death must surely follow. 



In both these cases the survivors would be only a small minor- 

 ity, and with such a decimation it would be astonishing if there 

 were any Porthesias left. 



Discussions of the selection theory never lead to an entirely 

 certain conclusion, but always to probabilities. I believe I have 

 shown that in our case the possibility of an explanation by 

 selection is very slight. It will naturally be even slighter if 

 we consider combined tropisms like those of the water scorpion 

 and the Cypris. Loeb has not concerned himself in any detail 

 with the problem of the purposeful nature of tropisms. But it 

 is not without amusement that we note that he, who regarded 

 the consideration of the question of selection as " a loss of time 

 in an idle play of words," should be forced to an ultradarwinian 

 conclusion by a logical analysis of his own theory. 



