EVOLUTION 203 



so widespread in all the higher plants and animals. Herit- 

 able variations are constantly arising in simple organisms, 

 as has been demonstrated by J ennings ^^"^ in Difflugia, and 

 it may be assumed that these are in part favorable and in 

 part unfavorable. The union of two individuals would 

 have the same chance of bringing together the greatest 

 number of favorable growth factors and the progeny 

 would thus be benefited, even though the mechanism fur 

 bringing this about is not as well organized as in the 

 higher forms. 



Some evidence of the possible importance of heterosis 

 in the establishment of sex may be obtained by the con- 

 sideration of an analogous phenomenon, double fertiliza- 

 tion among the angiosperms. In the gymnosperms the 

 embryo develops from the fertilized germ cell, of course, 

 but the endosperm which nourishes the young seedling is 

 gametophyte tissue. In the angiosperms the endospenn 

 as well as the embryo develops after a fertilization has 

 taken place. The conditions are slightly different, as a 

 fusion between two maternal nuclei occurs before tin? 

 union with the second male nucleus, but the essential 

 feature is the same as in the production of the embryo — 

 different hereditary materials are united when cross-fer- 

 tilization occurs. And in the same way that the embr^^o 

 and the resulting plant may be greatly benefited by cross- 

 fertdization, so also is the endosperm tissue increased in 

 amount as a direct manifestation of hybrid vigor. 



Nemec^^2 j^^g sought to account for endosperm hy- 

 bridization as an adaptation which results in a better 

 adjustment of the composition of the reserve food supply 

 to the needs of a hybrid embryo. The cross between some- 

 what different types results in an embryo which presum- 



