456 BIOLOGY IN RELATION TO OTHER NATURAL SCIENCES. 

 PHOTOTAXIS AND CHEMIOTAXIS. 



Considering- that every organism mnst have sprung from a unicellular 

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

 deferred epigenesis of 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 however 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 behavior of uni- 

 cellular 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 stinudi employed ::in 

 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 influ- 

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

 is termed, Phototaxin. The rod-like purjde organism called by Engel- 

 mann Bacterium plwiometricnm* is such a light-lover that if you place 

 a drop of water containing these organisms under the microscope, and 

 focus the smallest possible beam of liglit 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 color. If, instead of mak- 

 ing his trap of white light, he in-ojected on the field a microscopic 

 spectrum, Engelmann found that the rodlets showed their preference 

 for a spectral color, which is absorbed when transmitted through their 

 bodies. By the aid of a light trap of the same kind," the very well 

 known siiindle-shaped and flagellate cell of Euglena can be shown to 

 have a similar power of discriminating color, but its preference is differ- 

 ent. This familiar organism advances with its flagellum for":vards, 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 

 organisms to be observed are not colored, but bacteria of that common 

 sort which twenty years ago we used to call Bacterium termo, and 

 which is recognized as the ordinary determining cause of putrefaction. 

 These organisms do not care for light, but are great oxygen-lovers. Con- 

 sequently, if you illuminate with your spectrum a filament of a confer- 

 void alga, placed in water containing bacteria, the assimilation of 



* Engelmann, "Bacteriiun pbotometricum," Onderzoek. Physiol. Lab. Utrecht, vol. 

 Vii, p. 200; also Ueber Licbt-u. Farbenperceptiou nlederster Organismen, Pjtiiger's 

 Arch., vol. XXIV, p. 387. 



