105 



should be judged not only from the scientific, but also from the didac- 

 tic point of view. It is at this point that BEIJERINCK mentions his in- 

 tention to publish a review of the many experiments of this type 

 which already at that time were regularly carried out in his labor- 

 atory i). 



From that time on BEIJERINCK seems to have been fully aware of 

 the possibilities held out by the so-called "elective culture", and there 

 is no doubt that we owe to this awareness several of his most sensatio- 

 nal discoveries. The fundamental researches on oligonitrophilous 

 microbes, leading amongst other things to the discovery of Azotobacter 

 chroococcum, were a direct outcome of the enrichment principle. The 

 same can be said of the study made in collaboration with VAN DELDEN 

 "On a colourless bacterium, whose carbon food comes from the at- 

 mosphere", viz., Bacillus oligocarbophilus, and also of the studies on 

 the thionic acid bacteria, on the lactic acid bacteria, on Sarcina ven- 

 triculi, etc. 



Moreover, in several important papers by BEIJERINCK'S collabor- 

 ators full extension was given to this principle. We may refer in this 

 connection to the papers of VAN ITERSON on denitrifying bacteria and 

 on the bacteria which bring about the aerobic decomposition of cel- 

 lulose, to those of JACOBSEN on the bacteria which oxidize hydrogen 

 sulphide, sulphur etc., and to those of SOHNGEN on methane fermenta- 

 tion and on the bacteria oxidizing hydrogen, methane, kerosene, and 

 other hydrocarbons. 



By investigations of this character, BEIJERINCK and his school 

 have made a most thorough exploration of the microbe world. In 

 those years one specialized microbe was hardly discovered before an 

 announcement was made of the discovery of another specialized 

 organism with even more remarkable powers! 



It would be wrong to leave the impression that the elective method 

 owes its importance only to the fact that it enables the investigator 

 to isolate at any moment any desired type of microbes. BEIJERINCK 

 always .emphasized that the results obtained in the enrichment 

 experiments also throw considerable light on the microbial accumula- 

 tions occurring under natural conditions. In other words, these ex- 

 periments constitute an important contribution to the ecology of 

 micro-organisms. That herewith one can also get a clearer insight 

 into the role of these organisms in the successive processes which have 

 led to the formation of the earth's crust in its present aspect is in- 

 timated in several places in BEIJERINCK'S papers. Yet, it seems that 

 even nowadays geology is only beginning to awake to the importance 

 of microbial activities in the genesis of many deposits and ores. 



') Unfortunately BEIJERINCK has never accomplished this task. In 1907, however, a 

 booklet in the German language appeared under the title "Okologie, Anhaufungen 

 nach BEIJERINCK" by Dr. FERDINAND STOCKHAUSEN. The author who had worked for 

 some time in BEIJERINCK'S laboratory had undoubtedly been tempted to this pro- 

 duction by the oral expositions of BEIJERINCK. 



