FIELD NATURALISTS' CLUB. 31 



sionally we must accept Zopf's classification which is based on the 

 theory of pleomorphism. 



We must, however, clearly recognise that while a bacterium 

 can change its form it cannot change its nature, and be converted 

 from a harmless into a pathogenic form, as was asserted by 

 Buchner. This theory has been disproved by Koch and Klein. 



The most important practical question which arises with 

 reference to any micro-organism is whether it is pathogenic or 

 capable of invading and multiplying in living tissues, thus pro- 

 ducing disease in man or animals, or whether it is non-pathogenic 

 or saprophytic, i.e., only able to develop on dead tissue, and 

 therefore found in sloughs and discharges. To determine this 

 point Koch has laid down the following four postulates : — 



1. The micro-organism must be found in the blood, lymph or 

 tissues of the diseased man or animal. 



2. The micro-organisms must be isolated from the blood, 

 lymph or tissues, and cultivated on suitable media outside the 

 body. These cultivations must be carried on through successive 

 generations. 



3. A pure cultivation thus obtained must when introduced 

 into the body of a healthy animal produce the disease in question. 



4. In the inoculated animal the same micro-organism must 

 again be found. 



These postulates have been fulfilled in the case of certain 

 diseases which attack animals, such as anthrax, glanders, tuber- 

 culosis and malignant oedema. In human diseases there is an 

 obvious difficulty in the last two postulates which involve inocu- 

 lation of a healthy human subject. In such cases we have to be 

 content with experiments on animals, but here we are faced by 

 the difficulty that certain diseases such as leprosy are not found 

 in the lower animals. 



Lastly a few words may be said on the distribution of 

 bacteria outside the animal body. They are found in earth, air, 

 and water. 



Portions of garden mould taken from the surface and dropped 

 into sterilized culture fluid always infect it, producing a culture 

 of cocci and bacilli. In winter Koch has found that all organisms 

 are absent at a depth of one metre, in soil which has not been 

 recently disturbed, which does not consist largely of decomposing 

 material, and into which there is no unusual soakage of water. 



Recent researches have shown that bacteria play an important 

 part in the nitrification of soil. As these bacteria are chiefly 

 found near the surface, the necessity at once becomes apparent, 

 in agricultural operations, of frequently forking over the soil 

 round the roots of trees or plants. 



In air bacteria usually occur attached to particles of dust. 



