62 MICROBES AND TOXINS 



examples of mutation resembling those studied by the botanist 

 Hugo de Vries. In a pure culture of bacillus coli Massini 

 found that certain individuals suddenly acquired the power of 

 fermenting lactose, and that this property was transmitted to 

 their descendants. The attenuation of the anthrax vaccines of 

 Pasteur is an example of a property acquired and transmitted 

 hereditarily by the spore. 



Precisely because of this pleomorphism and plasticity of 

 bacteria, one must take care not to imagine mutations too 

 frequently. Occasionally modifications may be got which are 

 difificult to fix and which do not constitute true races. The 

 bacillus prodigiosus has been cultivated at 37'5° C, with- 

 out producing its red colour, for a series of generations ; but 

 after the thirty-fifth passage, when put again at 22° C, it re- 

 produced its pigment. Besides, mutation is not to be defined 

 simply as a sudden variation. There must be transmission by 

 sexual reproduction in addition. One could not speak of 

 mutation except in cases such as those of the anthrax bacillus, 

 where there has been suspected a sort of sexuality in the spore 

 forms. It is none the less true that the life of bacteria presents 

 numerous facts in accordance with the Darwinian laws. 

 " Bacteriology, like all branches of biology, has gained by the 

 application of the theory of evolution, and has made a fair 

 return by supplying the Darwinian theory with a striking 

 confirmation " (Metchnikoff). 



The Place of Bacteria in Classification. — 

 Leeuwenhoek, describing in 1683 in his Arcana naturae 

 detecta the micro-organisms of the mouth, which he observed 

 through lenses polished by himself, represented them as 

 animalculae. In 1838 Ehrenberg assigned them a place in his 

 work on the Infusoria, classing them with the Vibrios, which 

 he regarded as animals. But their evolution and their activities 

 indicate that the bacteria are lower plants. 



Are they to be classed among the fungi (moulds and yeasts) 

 or among the algse ? In the absence of chlorophyll, bacteria 

 resemble fungi, and they have long been called Schizomycetes, 

 i.e., fungi multiplying by transverse division. There are 



