THE PRESENT EVOLXJTION OF MAN — PHYSICAL 267 



not at all prevalent, that we are able to perceive that 

 any evolution has taken place. And here it may paren- 

 thetically 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 pai't of the specific interest of patho- 

 genic 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 phago- 

 cytes, 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 

 stiU susceptible — e. g. the microbes of measles. There- 

 fore 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 sUght as to 

 be wholly inappreciable, or which is non-existent owing 

 to concurrent retrogression. It is only when we con- 

 sider 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 



