§ 3] RHEOTAXIS 109 



against the current. The current controls the direction of 

 locomotion. 



The evidence that it is indeed the current is partly gained 

 by exclusion. It cannot be geotaxis, for if the current is 

 flowing upwards on any arm of the strip, the plasmodium 

 flows down. It can hardly be hydrotaxis, for the strip is uni- 

 formly saturated throughout. The action of light may be 

 excluded by shutting the whole apparatus in the dark, when 

 the same response occurs. When the direction of the current 

 in the strip is reversed, the movement of the plasmodium is 

 reversed also. Thus no other cause will explain the result but 

 that of the moving water. 



Satisfactory evidence that it is the current as such which acts 

 will not be forthcoming until it has been shown that other 

 fluids than water, e.g. oil, provoke a similar response. Until 

 such an explanation has been tried, it must remain uncertain 

 whether the phenomenon is not perhaps due to a difference in 

 the quality of the afferent and the efferent water. 



Finally, it must be mentioned that higher organisms, espe- 

 cially fish, are rheotactic. Whoever has seen fish ascending 

 streams from the sea in the spring has had this vividly 

 impressed upon him. Before some dam thousands of fish will 

 be seen, all facing the torrent of water against which they can 

 hardly hold their own. It is the current which determines 

 their position. They are responding to the direction of flow 

 of the waters. 



To recapitulate : In many non-living substances, especially 

 organic compounds, violent chemical changes (explosions) are 

 brought about by contact and especially by repeated vibra- 

 tions. S(f, too, in protoplasm, chemical change, exhibiting 

 itself in modified metabolism, frequently follows contact. The 

 explanation adapted to the non-living series of phenomena is 

 adapted to the living series also, — the molecules of the sub- 

 stance are complex, loosely associated, very unstable, so that 

 even a slight mechanical disturbance will serve to dissociate 

 their atoms. Protoplasm is a mixture of so many substances 

 that the whole mass does not become changed at once ; but 

 continued stimulation may eventually produce such wide- 

 spread changes as to lead to death. One of the most evident 



