78 INBREEDING AND OUTBEEEDING 



zygous hermaphroditic plants which are self-fertilized 

 naturally the only material having a critical value with 

 this mode of attack, if we except unicellular organisms 

 reproducing asexually have been made by a number of 

 biologists following the lead of Johannsen, 109 who opened 

 up the possibilities of this type of experiment. The re- 

 sults have always been the same. Characters are remark- 

 ably stable. They do change, but they change so rarely 

 that a more useful purpose is served by identifying the 

 physical unit factor with the mathematical factor unit, 

 than to assume without .justification that the physical 

 factor is constantly changing and must be described by 

 complex mathematical formulas using other hypothetical 

 units having no warrant for a physical existence. It is 

 true, two investigations by Jennings 107 and by Middle- 

 ton 142 have shown a seemingly more unstable condition in 

 the infusorians Difflugia coronata and Stylonychia pustu- 

 lata. But there are several reasons for not believing con- 

 clusions derived from data on these animals applicable to 

 the higher plants and animals in which our real interest 

 lies, without mentioning several technical points which 

 might lead to interpretations different from those given 

 by the authors. First, they are cases of asexual, not sex- 

 ual, reproduction. Second, the germ plasm of the infusoria 

 may not be insulated from the effects of environment as is 

 the germ plasm of the higher organisms. Third, measur- 

 able differentiation in these experiments sometimes took 

 such a number of generations that in man it would take 

 some 3000 years to produce like results. 



For these and other reasons which might be given, 

 could further space be devoted to the subject, we believe 

 there should be no hesitation in identifying the hypotheti- 



