MENDELISM AND OTHER METHODS OF DESCENT 20$ 



eludes all cases in which the parent organisms differ by two or 

 more characters which are preserved in the offspring. All species 

 in which definite diversities are maintained like those of sexes, 

 castes, dimorphism, polymorphism, etc., are examples of polar 

 inheritance, as are also the crosses of different varieties when 

 the parental diversities are repeated in later generations. The 

 general opinion has been that when two unlike individuals are 

 bred together their differences tend to average away or come to 

 an intermediate point in the offspring, but this is not true when 

 descent follows the methods of polar inheritance. 



The diverse characters of the parent organisms continue to be 

 represented in the perjugate gametes and the perjugate off- 

 spring, instead of being lost by intermediate or graded inheri- 

 tance or other more miscellaneous variations. In several forms 

 of polar inheritance equal numbers of the perjugate gametes 

 represent the divergent parental characters, but there is great 

 diversity in the relations assumed by the parent gametes, as 

 shown by diversity of behavior in the conjugate generation. 



Polar Inheritance in Dimorphic and Polymorphic Species. — 

 Two conditions of polar inheritance may be distinguished, those 

 in which the polarity is already established under conditions 

 of free interbreeding and those in which relations of polarity 

 are manifested when representatives of different varieties and 

 species are bred together. In the one case the polarity is alto- 

 gether natural, in the other somewhat artificial or experimental. 

 That there is any essential difference in the nature of the polarity 

 phenomena of the two series of cases, is scarcely to be claimed, 

 but it is very desirable to distinguish them in attempting to esti- 

 mate correctly the general significance of the polarity phenomena. 



Polar Inheritance of Sexual Differences [Sexual Di^nor- 

 ■phisni). — The most familiar phenomenon of polar inheritance is 

 that of sex. The parents are diverse, often by a large series of 

 definitely contrasted characters, and the offspring, both conju- 

 gates and perjugates, preserve all these contrasts. Sex-inher- 

 itance differs from Mendelism mostly in being a much more 

 specialized form of the same phenomenon of polarity in the ex- 

 pression of divergent characters. 



Semisexual Dimorphism. — Semisexual dimorphism is a 

 Proc. Wash. Acad. Sci., July, 1907. 



