ton and Pearson, formulated as statistical summaries of 

 certain phenomena of human inheritance taken en masse. 

 According to Galton's celebrated law of ancestral inheri- 

 tance, the qualities of any organism are determined to the 

 extent of a certain fraction by its two parents taken to- 

 gether as a mid-parent, that a smaller definite fraction is 

 contributed by the grandparents taken together as a mid- 

 grandparent, and so on to earlier generations. But Men- 

 del's Law has far greater definiteness, 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' Mutation Theory has already been explained 

 in an earlier address by Professor Richards. It is clearly 

 not an alternative but a complementary theory to Natural 

 Selection, the Germ-Plasm 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 accumulation of slight modifications or 

 fluctuations. The mutants like any other variants must pre- 

 sent themselves before the jury of environmental circum- 

 stances, which passes judgment upon their condition of 

 adaptation, and they too must abide by the verdict that 

 means life or death. 



From what has been said of these post-Darwinian dis- 

 coveries, the Lamarckian doctrine, which teaches that ac- 

 quired non-congenital characters are transmitted, seems 

 to be ruled out. I would not lead you to believe that the 

 matter is settled. I would say only that the non-transmis- 

 sion of racial mutilations, negative breeding experiments 

 upon mutilated rats and mice, the results of further study 

 of supposedly transmitted immunity to poisons that all 

 these have led zoologists to render the verdict of "not 

 proved." The future may bring to light positive evidence, 



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