No. 522] 



335 



group can be described by the same terms as used for the 

 animal group. 



Before proceeding farther in the comparison of plant 

 and animal responses, we must consider the relation of 

 responses in fixed forms to those in free-moving forms. 

 It is well known that botanists generally use the terms 

 "tropism" and "taxis" to distinguish movement in the 

 fixed organisms from that in the free-moving. Some zo- 

 ologists also, among them Verworn, observe the same 

 practise. The majority of zoologists, however, following 

 the lead of Loeb, omit the "taxis" designation wholly, 

 and include the movements of both fixed and free-moving 

 forms in the term "tropism." This is merely a ques- 

 tion of terminology, and need not occupy our time here. 

 It is more important to consider whether the behavior of 

 the fixed and that of the free-moving lower organisms 

 are enough alike to be regarded as fundamentally the 

 same. We may cite an analogy by saying that a traction 

 engine may one hour be pulling a load along a road, and 

 the next hour it may be stationary and operating a 

 threshing machine. To enable it to do the one or the 

 other requires but a slight alteration in the form of con- 

 nection of its parts, a slight change in its mechanism. 

 A swarm cell of the alga. JlotJirix, may alter its direction 

 under the application of a slight stimulus, and swim 

 toward the source of light. It may, a short time later, 

 fix itself to the substratum, and now bend as a fixed or- 

 ganism toward the source of light. One can readily be- 

 lieve that the two reactions are the same except for the 

 difference in the mechanism. 



Besides the evidence for the fnmbiim-nta] similarity of 

 sensitive processes in iixed and free moving organisms 

 furnished by the foregoing example, is the evidence pre- 

 sented by the quality of relation of stimulus and response 

 in the two kinds of organisms: Both kinds of organisms, 

 according to their structure, show simple positive or neg- 

 ative movements toward or away from the source of stim- 

 ulation; both kinds are sensitive to the same kinds of 



