268 THE PRESENT EVOLUTION OF MAN — PHYSICAL 



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 ob- 

 servation, 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 ex- 

 amination, 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 re- 

 sistant 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 coun- 

 tries. 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 



