210 S. O. MAST 



53-55, he considers as "the tropism theory," and without such 

 a statement his criticism is meaningless, for every method of 

 orientation described is in accord with one or another of the 

 fifteen theories presented. They are practically all tropisms in 

 one sense or another and 1 have nowhere even so much as 

 intimated that they are not. I have however pointed out in 

 manv instances that various methods of orientation do not 

 support certain prominent tropism theories. 1 am greatly dis- 

 appointed that my critic has used this term in such a loose way 

 after all the time and energy I spent in trying to show what 

 endless confusion has been caused by just such carelessness. 



In answer to two letters addressed to Professor Parker request- 

 ing first, a definition of the term tropism and second an explana- 

 tion of its meaning as used in his criticism he says in his first 

 reply: '" 1 frankly confess that I cannot give a clear definition 

 of the tropism theory or the theory of trial and error ", and in 

 his second: " The gist of the tropism idea as it lies in my mind 

 is that it is an orientation reaction dependent upon unequal 

 stimulation of two symmetrical parts of the organism's body." 2 



Since Parker admits that he cannot define clearly either the 

 tropism or the trial and error theory it is not surprising that 

 he concludes, as quoted above, that certain cases which illustrate 

 the one serve equally well to support the other. Hazy defini- 

 tions invariably lead to confused thinking. I may add however 

 that I have had no difficulty in understanding what Jennings 

 means by trial and error, and I assume that this is what my 

 critic implies by the " theory of trial and error." 



3. In the process of readjustment to a change in the direc- 

 tion of illumination, Stentor may either stop and turn abruptly 

 or it may turn gradually. In the gradual turning from the 

 source of light Parker says we have a " good instance of tropic 

 response." This conclusion like the one referred to above is 

 not surprising on the basis of a tropism theory that cannot be 

 defined clearly. But let us apply the definition Parker furnished 

 in his second letter and see what ground there is for it. By 

 referring to this definition quoted above it will be seen at once 

 that it can be applied strictly only to bilaterally or radially 



2 In a later communication Parker requests that it be made clear that in his 

 first reply he refers to :i definition which would be acceptable to a majority of 

 those using the term, and that in his second reply he gives a personal definition. 



