EVOLUTION AS A NATURAL PROCESS 147 



such phenomena are due, so to speak, to the rise to 

 the surface of a hidden stream of germ plasm that had 

 flowed for one or many generations beneath its accom- 

 panying currents. I believe that the law is replacing 

 more and more the laws of Galton and Pearson, formu- 

 lated as statistical summaries of certain phenomena 

 of human inheritance taken en masse. According to 

 Galton's celebrated law of ancestral inheritance, the 

 qualities of any organism are determined to the ex- 

 tent of a certain fraction by its two parents taken 

 together as a " mid-parent," that a smaller defi- 

 nite fraction is contributed by the grandparents taken 

 together as a mid-grandparent, and so on to earlier 

 generations. But Mendel's Law has far greater defi- 

 niteness, it explains more accurately the cases of 

 alternative inheritance, and it may be shown to hold 

 for blended and mosaic inheritance as well. 



De Vries's new " mutation theory" is clearly not an 

 alternative but a complementary theory to natural 

 selection, the Weismannian and Mendelian theories. 

 Like these last, it emphasizes the importance of the 

 congenital hereditary qualities contained in the germ 

 plasm, though unlike the Darwinian doctrine it shows 

 that sometimes new forms may arise by sudden leaps 

 and not necessarily by the slow and gradual accumula- 

 tion of slight modifications or fluctuations. The 

 mutants like any other variants must present them- 

 selves before the jury of environmental circumstances, 

 which passes judgment upon their condition of adapta- 

 tion, and they, too, must abide by the verdict that 

 means life or death. 



From what has been said of these post-Darwinian 



