149 



curring in a culture of Photohacterium luminosum is given as one of the 

 examples. 



Finally the term "variation" is reserved for those cases in which the 

 original form is maintained, whilst, now and then, individual cells are 

 thrown off with different properties which on the whole are likewise 

 constant and remain so. Only occasionally the new forms throw off 

 other variants, amongst which the normal form may occur as an 

 atavist. A detailed description of several examples of this variation 

 in the more restricted sense is given in the paper. 



The discussion which foliowed Beijerinxk's lecture, in which 

 discussion de Vries also took part, apparently induced Beijerinck 

 to add to his paper a foot-note in which he says to agree perfectly with 

 the opinion of de Vries that sudden variation — mutation — - is often 

 responsible for the origin of new species. However, he emphasizes that 

 this concept is not capable of explaining the adaptation which so often 

 is characteristic for the variation. 



In 1911, in the first meeting of the "Nederlandsche Vereeniging 

 voor Microbiologie", Beijerinck returned to the subject. The ex- 

 tensive paper which was published as a result of this, his presi- 

 dential address, is bef ore all remarkable for its wealth of observations 

 on the variation of several very dissimilar micro-organisms i). 



Even to-day any theory of microbial variation should take account 

 of the numerous experimental data collected by Beijerinck. 



For Beijerinck himself these observations formed an ample basis 

 for his theoretical considerations, which deviate in many respects 

 from his earlier ones. 



This time Beijerinck distinguished three types of microbial va- 

 riation, viz., modification, fluctuation and mutation. 



"Modification" is the variation which may occur, if a microbe is 

 brought under a certain set of external conditions, but which dis- 

 appears, either at once or after a few cell generations, as soon as the 

 primary conditions are restored. This form of variation is, therefore, 

 non-hereditary. "Fluctuation" is the term used for the hereditary 

 change which may take place under the influence of external con- 

 ditions, in so f ar as this change is characterized by the f act that all or 

 the great majority of the individual cells of a strain are subject to it. 

 In "mutation" the external conditions are of subordinate importance, 

 the principal factors are the internal conditions present in a relatively 

 small number of cells. 



However, since fluctuations also occur leap-wise and external con- 

 ditions are sometimes decisive for mutations as well, there is only a 

 difference in degree between the two latter types of variation. 



The main part of the paper is devoted to a minute description of the 

 variation phenomena observed with various microbial cultures. It is 



1) Folia Microbiologica 1, 1, 1912. 



