144 THE PHILOSOPHY OF BIOLOGY 



only physico-chemical phenomena. The embryologist, 

 on the other hand, studies the organism as a whole and 

 seeks to determine how definite forms are produced, 

 and how a change in the external conditions affects 

 the assumption of these forms. We have seen with 

 what little success the attempts to relate embryological 

 processes with physico-chemical ones alone have met. 

 In all studies of organic form mechanism has failed. 

 It is useless to attempt to press the analogies of 

 crystalline form, and the forms assumed in nature by 

 dynamical geological agencies. If the reader examines 

 these analogies critically he will see that they are 

 superficial only. 



We seem, however, to see in those actions of the 

 organism which are called " tropistic ' or " tactic," 

 reactions of a purely physico-chemical nature, and 

 starting with these as a basis a plausible theory of 

 organic movements on a strictly mechanistic basis 

 might be built up. 1 A " tropism " is the movement of 

 a fixed organism with respect to a definitely directed 

 external stimulus. This movement may be that pro- 

 duced by growth of its parts, or by the differential 

 contraction or expansion of its parts. A " taxis " we 

 may call the motion of a freely-moving organism in 

 response to the same directed stimuli. The move- 

 ments whereby a green plant turns towards the light 

 are called heliotropic, and those of its roots in the 

 perpendicular direction are called geotropic. The 

 motion of the freely-moving larva of a barnacle, for 

 instance, in swimming towards a source of light are 

 called " phototactic." 



In all these cases we have to think of the stimulus 

 as a " field of energy " in the sense in which physicists 

 speak of electric, or magnetic, or electromagnetic, or 



1 Many of Jacques Loeb's remarkable investigations point in this direction. 



