MENDELISM AND OTHER METHODS OF DESCENT 239 



polarity, instead of by the Mendelian theory of alternative 

 inheritance of character-unit particles. 



Mutations do not differ from JNIendelian hybrids in any essen- 

 tial respect, either at the time of their first appearance or in 

 later combinations. The preservation of the new character 

 by definitely reciprocal inheritance of expression-polarities is 

 favored by the same conditions of restricted descent which 

 induce the mutative variations. 



The analogy of the Mendelian phenomena, applied to varia- 

 tions induced by crossing, shows that new characters which come 

 to expression in the first or conjugate generation are not likely 

 to be permanent. Dominant variations can gain expression in 

 the second or perjugate generation, but recessive variations are 

 not shown before the third generation, and may not be brought 

 into expression until still later generations, unless the first per- 

 jugate generation is self-fertilized. As many variations of 

 economic value behave as recessives, this fact is of practical 

 significance in breeding experiments. 



Two distinct phenomena have been confused in the Mendelian 

 conception of inheritance, transmission and expression. The 

 failure of a character to secure expression does not indicate that 

 it has failed of transmission. Polarity, or reciprocal expression 

 inheritance of divergent parental characters, explains the phe- 

 nomena of Mendelism and related forms of descent without 

 requiring the assumption of pure germ-cells or of character- 

 unit particles. 



There is no evidence that normal transmission-inheritance is 

 a phenomenon involving the alternative admission or exclusion 

 of character-units, or that characters are transmitted as particles 

 or mechanisms. The process of transmission is independent 

 and separate from the process of expression, which often yields 

 polar or reciprocal results. This reciprocal polarity of expres- 

 sion-inheritance shows how new characters can be preserved 

 and thus contribute to the normal diversity of a species or grad- 

 ually transform it. Evolutionary advance can thus take place 

 without selective or geographical isolation. The general evolu- 

 tionary significance of Mendelism lies in its testimony to this 

 fact, and not in the theories of inheritance by character-units 

 and pure germ-cells. 



