THE LAW OF JOHANNSEN. 21 1 



between them. This is the law, comparable to our law of Jo- 

 hannsen. 



One of the results is, that in the absence of the resistance of 

 air, in the ideal conditions of a vacuum, bodies fall with equal 

 velocity. We may compare this result under ideal conditions, 

 which illustrates the law, to the fact, that selection is ineffect- 

 ive where the selected material remains homogeneous in re- 

 spect to the genes present for the duration of the selection- 

 experiment. In everyday life we observe, that the velocity 

 with which bodies fall, is influenced by their specific gravity, 

 and we observe bodies which do not fall at all, or fall away 

 from the earth. And these observations, which at first sight 

 look incompatible with the experiments under ideal conditions, 

 may be compared to the observation which we can make every 

 day, that selection changes the mode of nearly every character 

 selected. 



In the field of Physics, this difference between Newton's law 

 and the experiences of everyday life is certainly felt as un- 

 comfortable by beginners. But the fashion just now is all 

 against little experiments showing, that in a more or less com- 

 plete vacuum it is possible by accurate measurements to see 

 certain objects always arrive at the bottom a little ahead of 

 certain others. Not so in Genetics. We have to proceed with 

 the utmost caution. If we say, that the non-effectiveness of 

 selection in pure clones proves the qualitative stability of 

 genes, the result of selection- experiments within pure clones 

 are shown us, in which indubitable changes have been brought 

 about. 



We think there is a tendency on the part of many Genetici- 

 ans to see the importance of the work of Johannsen and others 

 with pure lines, in the direct application to our conception of 

 the nature of species, rather than in the light it sheds on the 

 genes. Johannsen 's conception of the existence of pure lines is 

 frequently by his followers, but more often by his critics, 

 exaggerated into a pure-line theory of evolution. We think, 

 that his work has a very important, but decidedly indirect 



