THE HISTORY OF THE SUBJECT 15 



protistae had been often observed, especially by Ehrenberg^ of 

 Berlin, well known for his researches on infusoria. Then 

 Engelmann, Pfeffer, Strassburger, Stahl, and many others 

 discovered and studied more carefully the facts concerning 

 chemotaxis, thigmotaxis, rheotaxis, geotaxis, phototaxis, etc., 

 of bacteria, motile spores, rhizopoda, and so on. The question 

 arose if one should regard this singular behavior of the unicellular 

 organisms as an expression of conscious sensations, discrimination 

 or will. This view was as determinedly denied on the one hand 

 as it was accepted on the other. Whilst even today certain 

 scientists still consider the reactions of the unicellular organisms 

 as a manifestation of conscious sensation, discrimination or 

 will, others look upon them as unconscious reflex reactions of 

 cell organism, taking place as purely mechanically as the spinal 

 cord reflexes of vertebrates. This divergence of opinion would 

 have practically no value for the development of our knowledge 

 of irritability had not here, as in the case of the relations between 

 the mental and physical processes in man, the view been enter- 

 tained with more or less fervor, that at some stage or other in 

 the chain of the purely physiological processes of responsivity, 

 an intangible factor had been introduced which was considered 

 as the essential "cause" of the peculiar reactions to stimuli. It 

 is not here the place to enter into the question if, and in what 

 degree, animal psychology may be a field of scientific research. 

 Even if one looks upon conscious processes as effects of stimula- 

 tion, in both lower animals and in man, in no case should one 

 assume them to be factors of an essentially different nature, 

 interrupting the chain of the mechanical reactions ; neither should 

 one consider the particular characteristic responses observed in 

 unicellular organisms as effects of non-mechanical "causes." As 

 a result, a mysticism, in reality quite foreign to it, would be intro- 

 duced into physiology. As a matter of fact the physiological 

 investigations for the tropic reactions of stimuli, which have 

 been carried out in great number since the end of the eighties, 

 have shown more and more clearly that this peculiar behavior 

 of unicellular organisms towards unilateral stimuli is produced 



1 Ehrenberg : "Die Infusionstiere als vollkommene Organismen." Leipzig 1838. 



