INTRODUCTION xi 



and repeatedly checked and tested. If the science of bacteriology is 

 to be built solidly, the two necessities of accumulating accurate facts 

 and making generalisations and deductions must proceed side by side, 

 the former being well established before the latter are accepted. It is 

 the danger of a new science that too much is expected of it. 

 Bacteriology, except in a few well-defined spheres, cannot yet stand 

 alone as reliable basis for legislation. The bacteriologist must be 

 content at present to serve as indicator rather than as olictator. The 

 detection, for instance, of certain bacteria in milk or in oysters is an 

 indication, and not an absolute proposition, of unsatisfactory dairying or 

 oyster culture. Common sense and a broad view of all the ascertain- 

 able facts must guide those whose business it is to apply the findings 

 of bacteriology to preventive measures. 



In the pages that follow, a large number of statements occur as to 

 the external circumstances and conditions affecting the life of bacteria, 

 and to understand these rightly and hold them in right proportion to 

 each other, it is necessary to bear in mind that many, if not most of 

 them, are of relative importance. They are of value, not as isolated 

 units, but as parts of a whole. It is their co-ordination, relativity, 

 and correlation which must be sought after. Again, the presence of a 

 diphtheria bacillus in the throat of a healthy man appears at first sight 

 to be a fact of absolute and critical importance until the life-history of 

 the bacillus is inquired into and determined, and the relation of the 

 healthy tissues to the performance of its function understood. The 

 bacteriologist and worker in preventive medicine can never afford to 

 neglect the inter-relationship which exists between the seed and the 

 soil. It is not wholly the one or the other with which he has to deal 

 as a practical man. It is the combination and the inter-action 

 between the two. If that principle, and the relativity of our know- 

 ledge of bacteria and the rdle which they play are borne in mind, 

 there is little to fear from a transition period. 



Whilst there can then be no doubt as to the advantage of a 

 wide dissemination of the ascertained facts concerning bacteria, 

 especially in relation to water, air, milk, and other foods, it must not 

 be forgotten that only patient and skilled observation, and 

 experimental research in well-equipped laboratories, can advance this 

 branch of science or indeed train bacteriologists. The lives of 

 Darwin and Pasteur adequately illustrate this truth. As the world 

 learns its intimate relation to science, and the inter-dependence 

 between its life and scientific truth, States and public authorities may 

 be expected more heartily to support science. 



