MENDELISM AM) OTHER METHODS OF DESCENT 1 99 



of avoiding this confusion of unlike conditions is to keep in 

 mind the fact that different kinds of crosses may differ, not 

 only in the particular differences or amounts of difference which 

 the parent organisms show, but also in the fact that the parents 

 may belong to different kinds of groups. 



Thus if there is indiscriminate individual diversity, among 

 the members of the parent group (as in a natural, freely inter- 

 breeding species) this diversity will not disappear at once, even 

 though care be taken to bring together as parents two indi- 

 viduals which seem to be closely alike. But by selection a 

 uniform type may be bred in which the traditional ideal of 

 heredity, that like produces like, may appear to be fully ex- 

 emplified. And yet if the narrow breeding be carried on with 

 persistence, mutative reactions toward greater diversity will 

 appear, and these sudden deviations from an established heredity 

 are even more different than were the normally diverse indi- 

 viduals of the original species. 



Mendelism belongs to another series of methods of descent, 

 in which the parents are diverse in a more definite manner, that 

 is, by discriminate or regularly established differences, like those 

 existing between the sexes of a species or between selected 

 varieties. Finally, there is the still more fundamental diversity 

 between species which have been separated long enough for 

 definitely divergent characters to be developed. 



SYNOPSIS OF METHODS OF DESCENT. 



Descent with unrestricted, indiscriminate diversity (heterism). 



Genetic variation in unrestricted descent (neism). 

 Descent with restricted diversity (heredity). 



Discontinuous variation in restricted descent (mutation). 

 Reappearance of ancestral characters (atavism). 

 Descent with discriminate intraspecific diversity (ropism). 

 Descent with integradation of differences (scalar). 

 Descent with preservation of differences (polar). 

 Polar inheritance in dimorphic species. 

 Sexual dimorphism. 

 Semisexual dimorphism. 

 Subsexual dimorphism. 



