20 PATHOLOGICAL MYCOLOGY. 



commencement of the process and its final issue, there was consider- 

 able risk of extraneous contamination. It is nevertheless unjustifiable 

 to urge the extreme difficulty of the quest as the probable cause of a 

 fallacious result ; and the position which one must therefore almost 

 of necessity occupy at present is one of expectancy, until more 

 conclusive investigations have been completed, and the whole 

 question put in such a position that its settlement may become a 

 possibility. The evidence on this point so far produced tends 

 strongly against the transmutation theory; but as yet no sufficient 

 mass has been brought forward on either side to render a final 

 decision possible. 



Requirements of Micro-Organisms. 



18. At this stage it will be well, shortly and in general terms, to state 

 what are the main vital necessities of a micro-organism which deter- 

 mine the relation it will take up to the various tissues or media in 

 which it may occur. Lying as it does on the borderland between the 

 animal and the vegetable worlds, its nature is of a but slightly differ- 

 entiated type, and its requirements are proportionately simple. An 

 analysis of its chemical constitution shows it to be composed of 88 

 per cent, of water, 2 per cent, of inorganic ash, and 10 per cent, of 

 organic compounds. This composition gives a general indication 

 as to what substances it will chiefly require to sustain its life and 

 development ; and it is found accordingly that its food must consist 

 very largely of watery elements. The organic compounds requisite 

 to its life are chiefly those rich in carbon and nitrogen ; ,and the 

 minerals which it specially needs are potassium and compounds of 

 phosphorus. 



Micro-organisms differ to a very great extent as to their dependence 

 upon, or independence of, the presence of free oxygen. So marked is 

 the former condition in certain genera — as, for instance, the moulds — 

 that they' do not grow actively except on free surfaces, where, they 

 can obtain oxygen directly from the atmosphere, or under such con- 

 ditions as permit of their readily obtaining it. Other genera, and 

 notably those of the yeast series, are capable of passing through all 

 the stages of their life-history without obtaining oxygen, except the 



