24 KEPOET— 1893. 



is proved to exist — of the psychologist to approacli the phenomena of 

 mind too exclusively from the subjective side^ — may mutually correct and 

 assist each other. 



Phototaxis and Chemiotaxis. 



Considering that every organism must have sprung from a unicellular 

 ancestor, some have thought that unless we are prepared to admit a 

 deferred epigenesis oi mind, we must look for psychical manifestations 

 even among the lowest animals, and that as in the protozoon all the 

 vital activities are blended together, mind should be present among them 

 not merely potentially but actually, though in diminished degree. 



Such a hypothesis involves ultimate questions which it is unneces- 

 sary to enter upon : it will, bowever, be of interest in connection with 

 our present subject to discuss the phenomena which served as a basis for 

 it — those which relate to what may be termed the behaviour of unicellular 

 organisms and of individual cells, in so far as these last are capable of 

 reacting to external influences. The observations which afford us most 

 information are those in which the stimuli employed can be easily 

 measured, such as electrical currents, light, or chemical agents in 

 solution. 



A. single instance, or at most two, must suffice to illustrate the in- 

 fluence of light in directing the movements of freely moving cells, or, as 

 it is termed, phototaxis. The rod-like purple organism called by Engel- 

 mann Bacterium photometricum ' is such a light-lover that if you place 

 a drop of v^ater containing these organisms under the microscope, and 

 focus the smallest possible beam of light on a particular spot in the field, 

 the spot acts as a light trap and becomes so crowded with the little 

 rodlets as to acquire a deep port- wine colour. If instead of making his 

 trap of white light, he projected on the field a microscopic spectrum, 

 Engelmann found that the rodlets showed their preference for a spectral 

 colour whicb is absorbed when transmitted through their bodies. By 

 the aid of a light trap of the same kind, the very well-known spindle- 

 shaped and flagellate cell of Euglena can be shown to have a similar 

 power of discriminating colour, but its preference is different. This 

 familiar organism advances with its flagellum forwards, the sharp end of 

 the spindle having a red or orange eye point. Accordingly, the light it 

 loves is again that which is most absorbed — viz., the blue of the spectrum 

 (line f). 



These examples may serve as an introduction to a similar one in 

 which the directing cause of movement is not physical but chemical. 

 The spectral light trap is used in the way already described ; the or- 



' Engelmann, ' Bacterium photometricum,' Onderzoeli. Physiol. Lab. Utrecht, vol. 

 vii. p. 200 ; also ' Ueber Licht- u. Farbenperception niederster Organismen,' PflUger's 

 Arch., vol. xsix. p. 387. 



