Of the annual saving of 234,955 lives, 64 per cent. was ascribable to reduced mortality from acute and chronic infectious diseases; and of the mortality under these headings nearly one-third was referable to respiratory diseases, the same amount to tuberculosis, one-seventh to scarlet fever, one-thirteenth to measles and whooping cough, the same amount to typhus and enteric fever, and one-sixteenth to diarrhœal diseases. The gain of life may be further illustrated by the following figures. During the 32 years, 1881 to 1912, over seventeen millions deaths occurred in England and Wales. Had the experience of 1871-80 continued throughout the subsequent years, the number of deaths would have been increased by close on four millions. _Specific Causation of Disease_ The preceding review will have made it clear that in the period of earlier slow sanitary reform, although much invaluable work was being done, it was in some measure a groping in the dark, a continuous search for further light while pursuing (or at least advocating in season and out of season) such cleansing and purification of man’s surroundings as were evidently needed, and such segregation of the infectious sick as could be secured in the absence of complete information of the cases of sickness. Happily in the case of Small Pox there was an additional effective protection in vaccination. With Pasteur’s discoveries was inaugurated a new era in sanitation; the general microbial origin of infectious diseases, inferred from his discoveries, leading to the conclusion that the chief source of disease to others is man himself, and that his surroundings in the main cause disease insofar only as they become a vehicle for conveying disease by direct inhalation of infected dirt (Sax. _drit_ = excrement), or by swallowing specifically infected foods. The importance of the sanitary engineer in securing pure water supplies and satisfactory sewerage continues. The sanitary inspector’s work in removing nuisances and accumulations, any one of which might be specifically contaminated,[4] and in controlling overcrowding and uncleanliness as well as in other respects, remains indispensable. But the brunt of guidance in the exact prevention of disease, especially of communicable diseases, must necessarily now fall on the epidemiologist, the vital statistician, and the laboratory worker. _Present Limitations of Epidemiology_ The epidemiologist must always remain the chief of these three, suggesting and arranging the details appropriate to each investigation, putting together the facts supplied by the two other workers and drawing legitimate conclusions. In conducting his inquiries and in searching for further light on obscure points, he will need to remember Simon’s remarks (Eighth Report of the Privy Council):