46 THE HUMANIZING OF THE BRUTE. 



Loeb grants that life-phenomena are ' ' not dependent 

 solely upon the external causes acting upon the organ- 

 ism at a given moment, but upon these and upon the 

 conditions present within the organism taken together; 

 and the latter conditions are in themselves variable." 



But this does not affect in any way the mechanical 

 character of his theory. The manifestation of helio- 

 tropism changes, but it remains pure and unequivocal 

 heliotropism. Thus "a large number of animals be- 

 come positively heliotropic, when they are left in the 

 dark for a long time. If they are brought into light 

 of sufficient intensity, they become negatively helio- 

 tropic after a time and this the more quickly the more 

 intense the light." "We do not therefore always 

 meet with simple conditions in analyzing the causes 

 which determine the voluntary movements of an ani- 

 mal; but however complicated they may be, the vol- 

 untary movements of animals are nevertheless, as our 

 experience indicates, always unequivocally determined 

 only by such circumstances as determine also the 

 movements of bodies of inanimate nature. ' ' 



On what facts does L,oeb base his theory? L,et us 

 give at least one characteristic example. 



The caterpillars of Porthesia chrysorrhoea pass the 

 winter in their nests in fruit trees and bushes, which 

 they leave as soon as it becomes warm. Then they 

 creep up to the tips of branches to the small buds 

 which serve as their food. Now, as Loeb expressly 

 states, it is merely positive heliotropism and negative 

 geotropism which compels the caterpillars to creep 

 upward, where they are held fast on the small buds 

 by contact-irritability. For you can make the cater- 



