6 BACTERIA. 



life-history of these organisms that any accurate information 

 has been obtained. 



Within quite recent years it has been observed that very 

 marked modifications of certain of the features may be ob- 

 tained by subjecting the micro-organisms to conditions 

 different from those under which they are ordinarily met 

 with ; and as the outcome of these observations the way 

 has been opened up for the study of the evolution, perhaps 

 not of the micro-organisms themselves, but of their specifiq 

 functional characteristics. It has been found, for instance, 

 that under certain conditions, yeasts that ordinarily set up 

 active alcoholic fermentation are no longer able to do so ; that 

 by continued cultivation outside the body and in certain special 

 media the anthrax bacillus loses much of its virulence ; ,whilst 

 the tubercle bacillus, the cholera bacillus, the organisms met 

 with in diphtheria and in other diseases, may, under certain 

 conditions of cultivation, lose their disease-producing power 

 in a most remarkable degree. On again being placed under 

 conditions of a more favourable nature these organisms 

 resume their pathogenetic or disease-producing properties. 

 The same variations are met with in the colour-forming 

 species of bacteria, according as the conditions under which 

 they exist are favourable or unfavourable to the accentuation 

 of certain functions of the protoplasm. 



It will be evident from a consideration of all these facts that 

 bacteria must be looked upon as being governed by much the 

 same laws which govern other plants and animals ; that they 

 are composed of protoplasm, the functions of which may be 

 modified in various ways, and the forms of which may become 

 more fully differentiated, and that where greater differentia- 

 tion takes place the modification of function is always in the 

 direction of greater specialization ; that the conditions suit- 

 able for the existence of the more lowly organized and 

 functionally less specialized organisms need to be less 

 specialized than do those that are necessary for the growth 

 of the more highly developed fungi ; and that, consequently, 

 the bacteria of disease and putrefaction are, with few excep- 

 tions, of a comparatively low form, although the functions 

 of the protoplasm of these less specialized bacteria become 

 more specialized as they dwell for a longer and longer 

 period under any special set of conditions, one phase or 

 power of the protoplasm being drawn out and developed in 



