MENDELISM AND OTHER METHODS OF DESCENT 2l5 



bred mutative varieties differ, and interspecific hybrids usually 

 do not show the same definite polarities of character-expression. 



Sterile or Mule Hybrids {Coiijuoate Hybridism). — Conju- 

 gate hybridism is a form of descent in which the parent gametes 

 are not able to accomplish a complete conjugation, though they 

 are able to unite in fertilization and build up a conjugate organ- 

 ism. Normal perjugate gametes are not formed, nor any per ju- 

 gate organisms. 



Conjugate hybridism may be illustrated by the mule and other 

 sterile crosses between the members of distinct species, but mu- 

 tative variations may also be sterile like conjugate hybrids. 

 Usually these sterile conjugate hybrids are definitely and uni- 

 formly intermediate between the two parents, as though there 

 were definite tensions which had yielded a uniform resultant. 

 Thus there seems to be rather less diversity among mules than 

 among horses. 



Fertile Hybrids between Species [Perjugate Hybridism). — 

 Perjugate hybridism is a form of descent in which gametes rep- 

 resenting different species are able to perform a complete con- 

 jugation, so that a perjugate generation is produced. 



There are many stages and gradations between conjugate and 

 perjugate hybridism, and of perjugate hybridism many different 

 degrees might be distinguished. Between some species perju- 

 gate hybrids are to be obtained only rarely and are then weak, 

 short-lived or sterile. Where the parental incompatibility is some- 

 what less, the perjugate generation, though more readily pro- 

 duced, shows a wide range of diversity like that of mutations 

 of narrow-bred varieties. The following generations of such 

 mutative hybrids may also be very unstable, but in plants capable 

 of vegetative propagation these mutative hybrids are often 

 valuable. Other species hybridize with greater freedom, and 

 show less abnormal diversity. Finally, som6 hybrids between 

 species appear to be nearly as normal and stable as crosses be- 

 tween related varieties of the same species. 



