CHEMOTAXIS 25 



chemicals is a purposive trait, although many forms do not 

 react away from solutions which are decidedly injurious. 

 According to Massart Polytoma uveUa may not be repelled 

 even by the strongest chemicals. Other organisms while 

 perfectly able to swim away, do not seem to be prepared for 

 meeting such a situation as the presence of injurious chem- 

 icals by any appropriate response. If specimens of Loxo- 

 phyllum mekagris and Paramoecium are placed together 

 under a cover-glass and a small drop of acid applied to one 

 edge the Paramoecia will avoid the acid, but the Loxophylla 

 move about helplessly, many of them swimming directly 

 into the strong acid, others which may be overtaken by the 

 acid showing no more tendency to swim away from it than 

 toward it. The result is that nearly all of the Paramoecia 

 in the region of the diffusing acid escape, while nearly all 

 of the Loxophylla perish. 



The general tendency of Paramoecium to collect in all lands 

 of weak acid and to react negatively to alkalies is prob- 

 ably of little service to the organism. Bacteria show a posi- 

 tive reaction to various substances which they never encoun- 

 ter under normal conditions and whose presence can scarcely 

 be of any benefit to them. On the other hand, the positive 

 reactions of several species to oxygen, as well as the negative 

 reactions of certain anaerobic species, are doubtless beneficial 

 modes of response. It is not improbable that hi many 

 cases the response to substances that are not beneficial is 

 the incidental result of the organization which causes posi- 

 tive responses to substances that are beneficial. For 

 instance, it may be advantageous to Paramoecium to be con- 

 stituted so as to react positively to weak solutions of carbon 

 dioxide, and its positive response to other weak acids may 

 depend upon the same peculiarity. While much of the 

 chemotactic behavior of lower organisms may be indifferent 



