246 GRAND STRATEGY OF EVOLUTION 



turning) and the definition of its field of opportunity 

 for self-constructive action. 



Here again, the dual sovereignty of life is expressed 

 in its own architecture, and in that of the external 

 world of action to which it responds, and in which it 

 has its opportunities, and its limitations. 



3. Tropic Action. Conduct often appears to be 

 the expression solely of an internal sovereignty, acting 

 independently of external conditions. 



But the sovereignty of the external world is clearly 

 seen under controlled experimental conditions, that is 

 when a given organism is subject to one predominat- 

 ing influence, all others, so far as possible, being neu- 

 tralized, removed, or duly accredited. 



External sovereignty is then .manifest in a definite 

 vital response, commonly called a tropism, which may 

 be immediately expressed in terms of motion, or loco- 

 motion, or less immediately in terms of growth. Many 

 kinds of tropism are recognized, such as geotropism, 

 heliotropism, chemotropism, etc. 



By means of these prearranged experiments, the 

 biologist seeks, as it were, to dissect the action of the 

 environment, or to analyze the behavior of an animal, 

 for much the same reasons, and in much the same 

 ways, that the chemist seeks to analyze, quantitatively 

 and qualitatively, a chemical compound; that is in 

 order more clearly to reveal its constituent actors. 



Under such specific conditions, vital structures may 

 definitely orient themselves to what, for the time be- 

 ing, is the dominant line of external action; or the or- 

 ganism moves, or grows, in very different ways in 



