MENDELISM AND OTHER IMETHODS OF DESCENT 2I9 



do not require so extensive an assumption to be made. It is 

 not necessary to suppose that gamete-formation is the reverse 

 of fertilization. The facts indicate only that the perjugate 

 gametes are definitely polarized or predisposed to the expres- 

 sion of one or the other of the divergent parental characters. 

 It may not be entirely inconceivable that pure germ-cells could 

 be formed on the plan of the Mendelian theory if the incompati- 

 bility of a hybrid were great enough to prevent mitapsis, but 

 germ-cells which remained pure in this manner would consti- 

 tute a very abnormal and exceptional phenomenon, like parthe- 

 nogenesis or apogamy, instead of being a general method of 

 descent. 



If the parental protoplasms separated, instead of completing 

 the fusion of the chromatin, the parental characters should 

 segregate in the original groupings and not be found in all 

 varieties of combinations as happens whenever the parents 

 differ by two or more Mendelian characters. The theory of 

 incompatibility is also excluded by the fact that the varieties 

 which afford the best examples of Mendelism are usually closely 

 related members of the same species, and the crosses are en- 

 tirely fertile. 



The Mendelian '* principle of gametic purity " has sometimes 

 been described very appropriately as "the law of disjunction", 

 since it rests on a denial of the fact of mitapsis, and holds that 

 no permanent fusion takes place between the gametes which are 

 brought together in Mendelian experiments. The presence in 

 a germ-cell of the element or unit representing one of the diver- 

 gent characters is supposed to exclude the unit which represents 

 the alternative character. 



But even the phenomenon of dominance, the primary fact of 

 Mendelism, shows that the representation of a character in a 

 germ-cell and the bringing of it to expression in an organism 

 are two entirely distinct processes. In building up the conjugate 

 structure the two gametes have a wide range of methods of 

 cooperation. In Mendelism the character of one gamete domi- 

 nates completely, so that the tendency of the other gamete to 

 the expression of a different characteristic is completely con- 

 cealed and held in abeyance. And yet Mendelian experiments 



