Page:George Archdall Reid 1896 The present evolution of man.djvu/279

Rh not at all prevalent, that we are able to perceive that any evolution has taken place. And here it may parenthetically be remarked, that we must not fall into the mistake, very apt to be made, of supposing that any disease which is now very mild in type, though very prevalent, has become so owing to a protective evolution against it. It may originally have been mild in type. It is obviously no part of the specific interest of pathogenic micro-organisms that the host they for the time inhabit should be destroyed, for when the latter perishes the microbes within him of necessity perish also. Their sole specific object is, of course, specific persistence, and this, as in the case of higher plants and animals, is attained in various ways. Some species of pathogenic micro-organisms, to protect themselves from the phagocytes, secrete a very virulent, a very poisonous toxin—e.g. the microbes of small-pox; others again exhibit great personal vigour—e.g. the microbes of tuberculosis; in each case the persistence of the species, not the death of the host, is the object; yet others exhibit neither very virulent toxins nor great personal vigour, but the persistence of the species is secured by a rapid passage from hosts that have become resistant to hosts that are still susceptible—e.g. the microbes of measles. Therefore we must not suppose of chicken-pox, for example, because it is now very mild though very prevalent, that necessarily it was once very fatal. We must only suppose, necessarily, since chicken-pox causes some deaths, that there may have been some evolution against it—an evolution which may be so slight as to be wholly inappreciable, or which is non-existent owing to concurrent retrogression. It is only when we consider diseases which are very deadly as well as prevalent—deadly at least to races that have had little experience of them—that we shall be able to detect evidence of considerable evolution. Of such diseases there are at