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.
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 attitude 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 recognized that these deviations do not indicate the incorrectness of Mendel's laws, but that they are merely the result of secondary and often minor complications; the latter investigators have from this standpoint made further fruitful discoveries. The rôle of the other group of investigators in this case has consisted, primarily, in an attempt to minimize the importance of Mendel's laws and thus to retard the progress of science.
The case is similar in the realm of tropisms. Tropisms and tropism like reactions are elements which make possible for us a rationalistic conception of the psychological reactions of animals on the basis of chemical 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 differently from what it does under the influence of light finds its explanation 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 essentially limited to the surface. Certain writers, however, leave this difference 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 say that tropisms are without significance. Animals in general are symmetrically built and the motor elements of the right and left