230 COOK 



As bearing upon the solution of these problems of descent, 

 the facts of Mendelism take on ver}' great interest and impor- 

 tance. As yet they have served only to obscure the issues be- 

 cause they have been interpreted as phenomena of incompati- 

 bility or " disjunction." The facts of Mendelism take on far 

 greater significance as soon as we cease to compare them to the 

 abnormal results of incompatible hybrids, and perceive the far 

 closer analogy with the alternative or polarized expression of 

 sexual differences. 



RELATION OF MENDELISM TO NORMAL DIVERSITY. 



Only the gratuitous assumption of ideal uniformity stands in 

 the way of a recognition of the fact that the normal tendency 

 of evolution is not toward ''identity of form and structure" 

 inside specific lines, but toward the development of diversity 

 among the members of species. Diversity between the members 

 of the same species is often accentuated by differences of envir- 

 onment, but there are many differences, both individual and 

 sexual which are independent of the environment, and which 

 continue to appear, without regard to uniformity or diversity of 

 environment. Mendelism, like sexuality, is a means of preserv- 

 ing a desirable diversity of descent inside the species. It is not 

 a phenomenon of incompatibility, but rather of avidity, tending 

 to strengthen conjugation by accentuating the unlikeness of the 

 gametes. 



Sexuality and Mendelism are both to be reckoned as phenom- 

 ena of normal diversity of descent, or heterism. Sexuality is 

 the more specialized form of heterism, because only sexually 

 diverse gametes are united in conjugation, whereas like gametes 

 can still conjugate in Mendelian descent, and thus miss the effect 

 of the desirable diversity which in sex-inheritance is fully pre- 

 served. But sexuality and Mendelism are very closely alike in 

 methods of inheritance. Each organism and each gamete rep- 

 resents the expression of one of a pair of contrasted characters, 

 but not both. There is no tendency to combine the characters 

 into an intermediate average. The series or scale of possible 

 variations is represented by the two extremities, while the inter- 

 mediate points come only rarely and abnormally into expression. 



