We have seen that certain forms of disease are due to the action of infective agents, which, obtaining.an entrance into the body, multiply there and give rise to certain phenomena. It is matter of frequent observation that different persons are very variously susceptible to diseases of this class, and it is a notable fact that inheritance plays an important part in determining the varying degrees of susceptibility. Race is an important factor in determining the susceptibility to this class of diseases.

Amongst the diseases due to pathogenic microbes perhaps the most widespread in the animal kingdom is anthrax, which is 'produced by the bacillus anthracis. The most marked differences are observable amongst different species of animals to the attacks of this microbe, from immunity in the frog to extreme susceptibility in the mouse and guinea-pig. But a similar difference exists amongst the races of animals belonging to the same species. Thus white rats are extraordinarily insusceptible as compared with common rats. A similar but less extreme difference is noted by Chauveau in the susceptibility of sheep. It is much more difficult to produce anthrax in Algerian sheep than in ordinary breeds, a larger dose being required. The racial peculiarities of this breed determine a striking difference in susceptibility. Pasteur found a similar difference in relation to the poison of chicken cholera amongst the different breeds of fowls. Ordinary fowls are highly susceptible, bat those of the Cochin China breed are so to a very minor degree.

In man similar racial differences of susceptibility are observable. In a certain sense the " plague of lice" is a serious disease, and it is highly infectious. According to Murray (quoted by Darwin) the different races of men present great differences in the species of pediculi which infest them, and when the parasites habitual to one race stray on to the bodies of persons of a different one they generally survive only a few days. A structural or physiological peculiarity of the skin connected with the differences of race determines the susceptibility to the attacks of the parasite.

Race has much to do with the susceptibility to Yellow fever. There is abundant evidence that negroes are almost entirely insusceptible to this disease, which is so virulent in white men. The immunity of the negro is not due to his having lived for generations in countries where yellow fever prevails, because it is manifest when negroes are brought from parts of Africa where yellow fever is unknown. The absence of susceptibility extends to persons partly of negro descent, generally in proportion to the amount of the negro in their constitution. Negroes seem also to have considerably less susceptibility to Malarial fevers than the white races.

On the other hand, negroes are more susceptible to Small-pox than Europeans, and it generally attacks them in a more virulent form. They are also more susceptible to Cholera.

We may infer, therefore, that along with differences of race there go peculiarities of structure and function which determine differences of susceptibility.

Family peculiarities are of the same nature as those of races, the latter being, probably, in their origin derived from the former. There are great and manifest differences among families in susceptibility to infective agents. There are many facts in existence which show that scarlet fever and diphtheria are special scourges in particular families. Again, tuberculosis of the lungs is acknowledged by almost all authorities to prevail in families, so much so that authors use the expression "family phthisis." It is difficult in the case of families to trace these susceptibilities through a sufficient number of generations, but such facts as those just indicated show that the peculiarities of structure which make family and individual distinctions are correlated to differences in susceptibility to the class of diseases under consideration.