70 The Mechanistic Conception of Life 



this may be accomplished by protoplasm in any form. For 

 the inheritance of instincts it is only necessary that the egg 

 contain certain substances — which will determine the different 

 tropisms — and the conditions for producing bilateral symmetry 

 of the embryo. The mystery with which the ganglion-cell has 

 been surrounded led not only to no definite insight into these 

 processes, but has proved rather a hindrance in the attempt to 

 find the explanation of them. 



It is evident that there is no sharp line of demarkation 

 between reflexes and instincts. We find that authors prefer 

 to speak of reflexes in cases where the reaction of single parts 

 or organs of an animal to external stimuli is concerned; while 

 they speak of instincts where the reaction of the animal as a 

 whole is involved (as is the case in tropisms). 



4. If the mechanics of a number of instincts is explained 

 by means of the tropisms common to animals and plants, and 

 if the significance of the ganglion-cells is confined, as in all 

 reflex processes, to their power of conducting stimuli, we are 

 forced to ask what circumstances determine the coordinated 

 movements in reflexes, especially in the more complicated ones. 

 The assumption of complicated but unknowTi and perhaps 

 unknowable structures in the ganglion-cells served formerly as 

 a convenient terminus for all thought in this direction. In 

 giving up this assumption, we are called upon to show what 

 conditions are able to determine the coordinated character of 

 reflex movements. Experiments on galvanotropism of animals 

 suggest that a simple relation may exist between the orientation 

 of certain motor elements in the central nervous system and 

 the direction of the movements of the body which is called 

 forth by the activity of these elements. This perhaps creates 

 a rational basis for the further investigation of coordinated 

 movements.^ 



1 Since this was written von Uexkuell found a law which will go far in explaining 

 the mechanism of coordination, namely, that a stretched muscle shows an increased 

 irritability while the contracted muscle shows a decreased irritabihty. Since 



