40 THE NATURE OF LIFE 



vary the horizontal one. We find that 

 the direction of growth of the root cor- 

 responds exactly to the parallelogram 

 of forces, that is, it obeys the forces act- 

 ing upon it precisely according to pre- 

 diction, and in such a manner as to jus- 

 tify the conclusion that it behaves as a 

 machine. 



Tropism is also found in animals. 

 Many animals are "the slaves of light": 

 they can not help turning their heads 

 toward the light and, if free to do so, 

 move toward it. In this way we may 

 explain what at first sight seems to be a 

 very complicated case of instinct. Cer- 

 tain caterpillars hatch out in the spring 

 and climb to the top of the branches of 

 trees where they wait until the food ap- 

 pears. This was regarded as a partic- 

 ularly mysterious case of instinct. It 

 was later shown by Loeb, however, that 

 they were forced to climb upward by the 

 irresistible attraction of light. Accord- 

 ing to his view the response is a tropism 

 and is purely mechanical. 



