THE SIGNIFICANCE OF TROPISMS 123 



This is, in my opinion, an unnecessary complication of simple relations 

 which in this case introduces a demonstrable error of animal physiology 

 into plant physiology. 



X 



The progress of natural science depends upon the discovery of 

 rationalistic elements or simple natural laws. We find that there are 

 two classes of investigators in biology, grouped according to their atti- 

 tude toward such simple laws or rationalistic elements. One seems to 

 aim at the denial of the existence of such simple laws and every new 

 case which does not fall at once under this law is an opportunity for 

 them to point out the inadequacy of the latter. The other group of 

 investigators aims to discover and not to disprove laws. When such 

 investigators have discovered a simple law which is generally applicable, 

 they know that an apparent exception does not necessarily overthrow 

 the law, but that possibly an opportunity is offered for a new discovery 

 and an extension of the old law. Mendel's laws have been brilliantly 

 confirmed in a number of cases. In some cases of deviations (from 

 these laws), however, it has not always been possible to recognize at 

 once the causes of the same. One group of investigators has recog- 

 nized that these deviations do not indicate the incorrectness of Men- 

 del's laws, but that they are merely the result of secondary and often 

 minor complications; the latter investigators have from this stand- 

 point made further fruitful discoveries. The role of the other group 

 of investigators in this case has consisted, primarily, in an attempt to 

 minimize the imj^ortance of Mendel's laws and thus to retard the 

 progress of science. 



The case is similar in the realm of tropisms. Tropisms and tropism T 

 like reactions are elements which make possible for us a rationalistic con- 

 ception of the psychological reactions of animals on the basis of chem- 

 ical mass action, and I believe, therefore, that it is in the interest of the 

 progress of science to develop further the theory of animal tropisms. 

 The fact that in an electric current the same animal often moves differ- 

 ently from what it does under the influence of light finds its explana- 

 tion for the observer conversant with physical chemistry in the fact that 

 the electric current causes changes in the concentration of ions within, 

 as well as upon the surface, while the chemical action of light is essen- 

 tially limited to the surface. Certain writers, however, leave this dif- 

 ference in the action of the two agents out of consideration and make 

 use of the difference in the behavior of certain organisms in response to 

 light and to the electric current, to assert that it is not permissible to 

 speak of tropisms as being governed by general laws ; in other words, 

 they sa} r that tropisms are without significance. Animals in general 

 are symmetrically built and the motor elements of the right and left 



