218 The Mechanistic Conception of Life 



the specifically lighter substance comes in contact with the 

 sea-water (or gets near the surface of the cell) the growth of a 

 stem is favored; while contact with the sea-water of the 

 specifically heavier of the substances, will favor the formation 

 of roots. 



VI. the experimental control of animal instincts 



a) Experiments on the mechanism of heliotropic reactions in 

 animals. — Since the instinctive reactions of animals are as 

 hereditary as their morphological character, a discussion of 

 experiments on the physico-chemical character of the instinctive 

 reactions of animals should not be entirely omitted from this 

 sketch. It is obvious that such experiments must begin with 

 the simplest type of instincts, if they are expected to lead to 

 any results; and it is also obvious that only such animals must 

 be selected for this purpose, the reactions of which are not 

 complicated by associative memory or, as it may preferably be 

 termed, associative hysteresis. 



The simplest type of instincts is represented by the purpose- 

 ful motions of animals to or from a source of energy, e.g., light; 

 and it is with some of these that we intend to deal here. When 

 we expose winged aphides (after they have flown away from the 

 plant), or young caterpillars of Porthesia chrysorrhoea (when 

 they are aroused from their winter sleep), or marine or fresh- 

 water copepods and many other animals, to diffused daylight 

 falling in from a window, we notice a tendency among these 

 animals to move toward the source of light. If the animals are 

 naturallv sensitive, or if thev are rendered sensitive through the 

 agencies which we shall mention later, and if the light is strong 

 enough, they move toward the source of light in as straight a 

 line as the imperfections and peculiarities of their locomotor 

 apparatus will permit. It is also obvious that we are here 

 dealing with a forced reaction in which the animals have no 

 more choice in the direction of their motion than have the iron 



