184 THEORY OF EVOLUTION 



or slower rates of division. His observations 

 seem to show that his selection separated two 

 such lines that came from the same original 

 stock. The rapidity of the effects of selection 

 seems to preclude the explanation that pseudo- 

 parthenogenesis has complicated the results. 

 Nevertheless, the results are of such a kind as 

 to suggest that they were due to selection of 

 vegetative (somatic) differences and that no 

 genetic change of factors was involved, for his 

 conclusion that the rapidity with which the 

 effects gained by long selection might be sud- 

 denly reversed when selection was reversed is 

 hardly consistent with an interpretation of the 

 results based on changes in the "potencies" of 

 the factors present. 



Equally striking are the interesting experi- 

 ments that Jennings has recently carried out 

 with Difflugia (fig. 91). This protozoon se- 

 cretes a shell about itself which has a charac- 

 teristic shape, and often carries spines. The 

 opening at one end of the shell through which 

 the protoplasm protrudes to make the pseudo- 

 podia is surrounded by a rim having a charac- 

 teristic pattern. The protoplasm contains 



