91 



the mutation theory is more clearly expressed in the following footnote 

 of his pubhcation than in the text. 



"I perfectly agree with Professor de Vries, that the origin of 

 species should often be sought in the almost suddenly produced 

 variants, or mutants, as he calls them. This is also the conclusion 

 to which Galton has come regarding the races, and to which he 

 referred repeatedly since 1892, the last time, as far as I know, in 

 Nature, vol. 58, p. 274, 1898, in these words: "I have frequently 

 insisted that these sports or "aberrances" (if I may coin the word) 

 are notable factors in the evolution of races. Certainly the successive 

 improvements of breeds of domestic animals generally, as in those of 

 horses in particular, usually make fresh starts from decided sports 

 or aberrances and are by no means always developed slowly through 

 the accumulation of minute and favourable variations during a long 

 succession of generations". iVlong quite distinct ways Galton, de 

 Vries, and myself, have thus arrived at the same conclusion re- 

 garding the probable origin of many races and species. But the 

 great difficulty which lies in the explanation of adaptations, has 

 not been removed, neither by Galton's "aberrants" de Vries' 

 "mutants", nor my "variants"." 



The"Proceedingsof the Academy" report in a few words that this 

 lecture was foliowed by a discussion between Professor HuGO de 

 Vries and the speaker, in which Professor Hubrecht also took part. 

 Tradition has it that in this discussion the opinions were sharper op- 

 posed than might be thought from the report in the "Proceedings". 



A hint of the extent of the differences might also be gained from 

 the f act that Beijerinck avoided the use of the word "mutation" 

 until 1912. It was in the title of his extensive study on "Mutation bei 

 Mikroben" which appeared in that year i), that he joined in the use 

 of the word. In this treatise also, prospects are opened for the general 

 problem of heredity. A few citations from Beijerinck's study may 

 illustrate this. 



"Fluktuation und Mutation sind dem Grade nach verschieden. Bei 

 der ersten sind die Sprünge kleiner wie bei der zweiten ; die Aussen- 

 bedingungen sind beim Zustandekommen der Fluktuation, die Innen- 

 bedingungen bei der Mutation überwiegend". "Nach der Genentheo- 

 rie kann angenommen werden, dass sowohl bei der Mutation wie beim 

 Atavismus Progene in aktive Gene, und umgekehrt Gene in Progene 

 verwandelt werden". "Dass wahrhaft neue Gene bei der Mutation 

 jemals gebildet werden, ist nicht erwiesen, weder bei den Mikroben 

 noch bei den Pflanzen und Tieren. Wenn dieses der Fall zu sein scheint 



so ist doch viel wahrscheinlicher, dass die Progene. . . . schoninder 



Stammform gegenwartig war und durch Atavismus erwecktwürde". 



Finally we mention the remarkable pubhcation which is entitled 



>) Folia Microbiologica 1, 1-97, 1912 {Verzamelde Geschriften 5, 25-88). 



