INTRODUCTION. 5 



the conditions under which a special bacterium would grow. 

 It could be transferred from medium to medium ; it could 

 be placed under aerobic and anaerobic conditions ; or it 

 could be plied with antiseptic or other special reagents, and 

 its behaviour under varying conditions or in the presence of 

 these material could be accurately observed. There was no 

 longer any danger of one form being mistaken for the de- 

 velopmental stage of a totally different organism, and much 

 confusion as regards classification was gradually done away 

 with, though the effects of the observations made on impure 

 cultivations still make themselves felt in the obscurity 

 that enshrouds certain groups which are still met with 

 even in some of the better known classifications. As 

 these pure cultivations were obtained it gradually became 

 more apparent that, for practical purposes, it would be 

 necessary to base our classification on certain specific cha- 

 racters of the organism ; and as in all processes in which 

 bacteria play a part they are usually met with in a single 

 and special form, it was originally found advisable to take 

 these special forms as the diagnostic features in making such 

 classification. The features of form and size, however, were 

 merely superficial characters, and as a more careful study 

 of bacteria was made, especially by the French school, 

 it was found that the biological characters afforded a much 

 more satisfactory basis of classification, especially, however, 

 when taken in conjunction with the morphological features ; 

 so that the media on which they grow, the products to 

 which they give rise, the methods in which they cause the 

 breaking down of dead or living protoplasm, the actions and 

 reactions that take place between them and living animal 

 and vegetable cells, and the effects upon them of organic 

 and inorganic antiseptic substances, are now all taken into 

 account in drawing up any classification. Thus we have 

 yeasts that cause fermentation and yeasts that do not, whilst 

 nearly related to them are ascomycetes that are essentially 

 parasitic in their character ; we have micrococci that set up 

 urea fermentation, micrococci that form pigment, others 

 that give rise to suppuration or that apparently assist in the 

 production of diphtheria ; and so on, throughout the whole 

 of these low vegetable forms. The appearance under the 

 microscope can, of course, give us no information on many 

 of these points, and it is only by a most careful study of the 



