98 BIOLOGICAL LECTURES. 



into a preparation of Paramecia, they soon collect closely about 

 it and swim in circles around it without leaving it. 



It was then proved, by introducing the Paramecia into a 

 solution of rosol, which is decolorized by carbon dioxide, that 

 these Infusoria excrete a distinctly appreciable amount of this 

 substance, which diffuses into the surrounding water. When- 

 ever, therefore, a very few Paramecia get together, an active 

 solution of carbon dioxide is soon formed, and the region be- 

 comes at once a center of attraction for the Paramecia. A 

 most complete correspondence was demonstrated between the 

 diffusion of the COg into the water and the distribution of the 

 Paramecia in groups, and all the phenomena exhibited by 

 the (apparently) spontaneous collections of Paramecia could 

 be exactly imitated by introducing COg into the slide. 



Thus these collections of Paramecia give no indication of 

 " social instinct," but are merely the expression of positive 

 chemotaxis on the part of the animals toward a certain sub- 

 stance. In the same way all the seemingly complex activities of 

 these creatures may be reduced to simple factors, so that there 

 seems no evidence to indicate the possession by them of psychic 

 powers of anything more than the most elementary character. 



We may proceed then to a closer analysis of the apparent 

 attractions and repulsions — chemotaxis, thermotaxis, and the 

 like ; it is from a study of these that light is to be gained on 

 the problems first proposed. We shall first consider chemo- 

 taxis. 



The fact that animals and plants are attracted by certain 

 chemical substances and repelled by others is of course well 

 known for a large number of organisms. As to the essential 

 nature of this phenomenon, opinions differ. As pointed out 

 above, some hold that chemotaxis is the direct expression of 

 chemical affinity or repulsion between the living protoplasm 

 and the chemical. Le Dantec {La matih'e vivante, pp. 51, 52) 

 gives geometrical figures illustrating the action between the 

 surface of a free cell and a chemical substance diffusing in the 

 surrounding water, demonstrating in mathematical form that 

 as a result of this action the cell must move either toward or 

 away from the center of diffusion of the chemical. The motion 



