least two—tuberculosis and malaria. We will consider them first, and afterwards pass to the consideration of other less prevalent or less deadly diseases, when we shall find that the evidences of evolution, though less clear and unequivocal, are often clear and unequivocal enough.
It must be borne in mind, however, that never before now, at least so far as I am aware, have diseases been considered from the standpoint of the evolutionist, and that therefore there is no literature bearing on the subject to which we can appeal. As I have already indicated, the attention of biologists has hitherto been directed solely to changes of form sufficiently gross to be perceptible to the naked eye, or under the microscope, or to mental changes. This other, this vastly important line of evolution, seems to have entirely escaped observation, and therefore I can offer the reader such proofs only as are to be found in books devoted to quite different objects, when, while accepting the facts furnished by various authorities, I shall often be obliged to dispute their inferences, since these have been drawn without reference to man's evolution in relation to zymotic disease. Before commencing our detailed examination, it will, however, be well to seek and note the explanation of two or three interesting phenomena.
Children are usually much less resistant to zymotic disease than adults of the same race. Thus while the adult negroes of the West Coast of Africa are very resistant to malaria, numbers of their children perish of it, though to nothing like the same extent as do the children of people immigrant from non-malarial countries. We might suppose that the superior immunity exhibited by adults is due solely to the fact that, in the presence of any prevalent zymotic disease, the weak against it perish early in life, leaving the more resistant to survive, and no doubt this to some extent explains the facts; but that it does not entirely do so
