No. 531] GENOTYPE CONCEPTION OF HEREDITY 159 



into Mendelian terms by the observations and experi- 

 ments of Punnett, de Meijere and others. This strong- 

 hold of the united Lamarckism and select ion ism has now 

 been conquered for Mendelism, i. e., for the genotype 

 conception. 



The genotype-conception here advocated does not pre- 

 tend to give a true or full " explanation " of heredity, but 

 may be regarded only as an implement for further critical 

 research, an implement that in its turn may be proved to 

 be insufficient, unilateral and even erroneous — as all 

 working-hypotheses may some time show themselves to 

 be. But as yet it seems to be the most prosperous leading 

 idea in genetics. 



Heredity may then be denned as the presence of iden- 

 tical genes in ancestors and descendants, or, as Morgan 

 says in full accordance with this definition: "The word 

 heredity stands for those properties of the germ-cells 

 that find their expression in the developing and developed 

 organism." 



And now it is time to end this communication, too long 

 for its real contents, but too short for the importance and 

 diversity of the great problem of heredity. 



In concluding this address I must highly emphasize the 

 eminent merits of Hugo de Vries. His famous book "Die 

 Mutationstheorie," rich as well in positive indications as 

 in ingenious views, has been the mediator for the new and 

 the old era in genetics. This monumental work is a land- 

 mark in the progress of our science. Like the head of 

 Janus it looks at once forward and backward, trying to 

 reconcile — at least partly— the antagonistic ideas of con- 

 tinuity and discontinuity in evolution and heredity; hence 

 a great deal of the charm of De Vries »s work. But just 

 these qualities have made the work of De Vries too 

 eclectic for the stringent analytical tendencies of modern 

 genetics — a tendency which has in recent years found a 

 true home in American science. 



