MR. PRESIDENT,-I would first of all express to you, Sir, and the Censors of the College my appreciation of the honour you have done me in electing me to deliver the Horace Dobell lecture. I feel, however, that it will be impossible for me to do justice to the fine spirit which actuated the founders of the lectureship during their lives' work. To successfully interfere in bacterial diseases of man and animals it is most important to know accurately the lifehistory of the parasite outside the

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... -how it leaves the old host, how it enters a new host, and where and how it passes the intervening period. If we are in possession of these facts our efforts in the struggle can at any rate be intelligently directed and concentrated against the most vulnerable point of the enemy. The exercise of an adverse influence upon bacteria, once they have gained a footing in the body, has hitherto not been generally successful, although recent developments in chemotherapy have shown that there is ground for optimism in this direction also. In the early days of bacteriology, before the large number of pathogenic bacteria had been discovered and their peculiarities had been studied, it was generally assumed that bacteria once let loose from the body survived long periods upon inanimate objects and in water, soil, &c. ; a,nd efforts at prevention were directed to an indiscriminate disinfection of the belongings and surroundings of the patient. Some bacteria do, indeed, possess considerable powers of survival under such circumstances, and I would not be understood to deprecate measures of general disinfection. The majority of pathogenic organisms, however, are fortunately delicate creatures, and rapidly succumb to such adverse influences as drying and sunlight. Another factor which must not be lost sight of is that many organisms which have acquired by selection the property of withstanding the adverse influence obtaining in the animal body fare ill in the competition with hardy saprophytic colleagues outside the body, and if certain individuals do survive, these, whilst acquiring the capacity to live upon extra-corporeal nutriment, diminish at the same time in pathogenicity. In other words, the conditions select a strain tending more and more towards saprophytism and away from parasitism. For these reasons it behoves us to be especially on the look out for any machinery through the agency of which bacteria may be rapidly conveyed from sick to healthy, and their travels made easier for them. In not a few diseases the infective agent usually passes almost directly from patient to patient-such are syphilis, plague-pneumonia, diphtheria, measles, scarlet fever, smallpox, and numerous parasitic skin diseases. The spread of such highly infectious diseases can be dealt with by isolation and segregation of contacts, for their range is small. Of recent years much attention has been bestowed upon the study of the r6le of insects in the transmission of disease. It is not my intention to deal with these disccveries as to the essential part played by insects in the transmission of protozoal parasites made by Smith and Kilbourne, Bruce, Ross ; Reed, Carrol, and Agramonte in the case of the transmission of Texas fever, nagana, sleeping sickness, malaria of birds and animals, and yellow fever respectively. It may, however, be pointed out that it was these researches which focused attention upon the possibility of bacterial diseases being conveyed by a similar agency. In the latter case the insect plays a less essential part, and no particular phase in the life-history of the parasite takes place within it. I have accordingly, following Ioo.^ A121Z'> a suggestion of my friend Colonel Alcoclc, employed the term "porters" to describe this passive i6'e of insects in the spread of contagion. HOUSE FLIES. A deal of attention has been paid to flies lately, in view of their possible influence in the dissemination of infection, more particularly in the case of such diseases as cholera, typhoid fever, and infantile diarrhoea, where the infective agent escapes from the intestine and new infections are taken in by the mouth. The first question is, Can the fly convey infection ? Before referring to the numerous experiments which have afforded an answer, I will briefly refer to those points in the lifehistory, structure, and habits of the house fly which are of assistance in appreciating how it may play such a r6le. These subjects have been submitted to careful inquiry during the last few years, particularly in America and this country, and, thanks to the observations of Lowne