Nowhere has preventive medicine achieved greater triumph than in the extermination of certain micro-organisms which gain access to the body and cause the febrile class of diseases, such as small-pox, measles, typhoid fever, and very many others. At present none of these micro-organisms can be said to be extinct, but they have in some cases been banished to distant parts of the globe, and in other cases the conditions suitable to their existence, and the means of their propagation are so well understood that their banishment is being systematically and successfully carried out, so much so that a disease such as small-pox, which at one time headed the list of fatal diseases, does not come in the category of anxieties of the mother of to-day; and pyæmia and puerperal fever, which twenty years ago were at times dreaded scourgers in most hospitals, now occur only from culpable and punishable neglect. _The Reproductive Cells are as a Rule unaffected by them._ A short study of these diseases should well repay us, showing as it will do that in by far the greater number of cases these severe constitutional derangements produce no effect upon the reproductive cells; that they are in fact incapable of producing a change that will be hereditarily transmitted. We shall learn, moreover, the part that they have played, and can play, in producing racial change by selection. The micro-organisms of disease are of many varieties, and each variety is capable of setting up its own peculiar disturbance. The disturbance set up by one kind we call small-pox, that arising from another kind, cholera, and so on. Now, the very curious point comes out that in most of these diseases, although the composition of the blood is profoundly altered, and many of the tissues undergo marked change, this change, fortunately for us, is quite of a temporary character, and when the attack is over there is only one test which will enable us to say that the body is not just in the same condition as it was before. This test is that it cannot be infected again for a long time, if at all, by the same micro-organism.