40 The Mechanistic Conception of Life 



those on the other side. The result of such an inequality of the 

 action of symmetrical muscles of the two sides of the body is 

 a change in the direction of movement on the part of the animal. 



The change in the direction of movement can result either 

 in a turning of the head and, in consequence, of the whole animal 

 toward the source of light, or in a turning of the head and the 

 animal in the opposite direction. The structure of the central 

 nervous system is segmental and the head segments generally 

 determine^ the behavior of the other segments with their acces- 

 sory parts. 



In the winged aphids the relations are as follows: Suppose 

 that a single source of light is present and that the light strikes 

 the animal from one side. As a consequence the activity of 

 those muscles which turn the head or body of the animal toward 

 the source of light will be increased.- As a result the head, and 

 with it the whole body of the animal, is turned toward the 

 source of light. As soon as this happens, the two retinae 

 become illuminated equally. There is therefore no longer 

 any cause for the animal to turn in one direction or the other. 

 It is thus automatically guided toward the source of light. 

 In this instance the light is the ''will" of the animal which 

 determines the direction of its movement, just as it is gravity 

 in the case of a falling stone or the movement of a planet. The 

 action of gravity upon the movement of the falling stone is 

 direct, while the action of light upon the^direction of movement 

 of the aphids is indirect, inasmuch as the animal is caused 

 only by means of an acceleration of photochemical reactions 

 to move in a definite direction. 



1 Loeb, Comparative Physiology of the Brain and Comparative Psychology, New 

 York and London, 1900. 



' If two sources of light of equal intensity are at an equal distance from the 

 animal, it will move in a direction at right angles to a line connecting the two 

 sources of light, because in this base both eyes are similarly influenced by the light. 

 Herein, as Bohn has rightly said, the machine-like heliotropic reaction of animals 

 differs from the movement of a human being toward one of two sources of light, 

 the movement in the latter case not being determined by heliotropism. 



