390 HEREDITY AND ENVIRONMENT 



and not in all directions, selection can act only 

 by sorting out mutations, but can do nothing 

 to produce them. In general fluctuations are 

 due to environment and are not inherited, 

 therefore they concern development rather 

 than heredity, developed characters rather than 

 inheritance factors. There is much evidence 

 that inheritance factors are relatively stable 

 and that when they change, they undergo a 

 complete change or mutation comparable to 

 what occurs in a chemical reaction. As we 

 have seen it is theoretically possible to explain 

 almost all phenomena of inheritance on this 

 basis even including Castle's results. 



Quite recently Jennings has shown that con- 

 tinued selection in one direction does, appar- 

 ently, shift the mode of certain characters in a 

 pure line of asexually reproducing Difflugia, 

 and Middleton has found the same to be true 

 of Stylonychia. These results differ totally 

 from Jennings' earlier work on Paramecium, 

 which has recently been repeated and con- 

 firmed by Ackert. It is a hard thing to believe 

 that different organisms differ irreconcilably in 

 so fundamental a matter and it seems much 



