xii INTRODUCTION 



Patrick Geddes declares, *' forms part of the general social 

 evolution." Certainly its recent rapid development forms a 

 remarkable feature in the practical science of our time. Not 

 only in the diagnosis and treatment of disease, nor even in 

 the various applications of preventive medicine, but in ever- 

 increasing degree and sphere, micro-organisms are recognised 

 as agents of utility or otherwise no longer to be ignored. 

 They occur in our drinking water, in our milk supply, in the 

 air we breathe. They ripen cream, and flavour butter. They 

 purify sewage, and remove waste organic products from the 

 land. They are the active agents in a dozen industrial 

 fermentations. They assist in the fixation of free nitrogen, 

 and they build up assimilable compounds. Their activity 

 assumes innumerable phases and occupies many spheres, 

 more frequently proving themselves beneficial than injurious. 

 They are both economic and industrious in the best biological 

 sense of the terms. 



Yet bacteriology has its limitations. It is well to recognise 

 this, for the new science has in some measure suffered in the 

 past from over-zealous friends. It cannot achieve everything 

 demanded of it, nor can it furnish a cause for every disease. 

 It is a science fuller of hope than proved and tested know- 

 ledge. We are as yet only upon the threshold of the matter. 

 As in the neighbouring realm of chemistry, it is to be feared 

 that bacteriology has not been without its alchemy. The 

 interpretations and conclusions which have been drawn from 

 time to time respecting bacteriological work have led to 

 alarmist views which have not, by later investigation, been 

 fully supported. Again, the science has had devotees who 

 have fondly believed, like the alchemists, that the twin secret 

 of transmuting the baser metals into gold and of indefinitely 

 prolonging human life Avas at last to be known. But neither 

 the worst fears of the alarmist nor the most sanguine hopes 

 of the alchemist have been verified. Science, fortunately, 

 does not progress at such speed, or with such kindly accom- 



