THE PRESENT EVOLUTION OF MAN — PHYSICAL 227 



immune under any circumstances ; others are ordinarily 

 immune, but are liable to be attacked when from any 

 cause their vitality is lowered, Avhen their johagocy tes are 

 less fit for the battle ; others again seem quite unable, 

 even under the most favourable circumstances, to resist 

 infection ; their phagocytes succumb easily, and in them 

 the disease usually runs a rapid course, and systemic 

 death quickly ensues. 



The fact that, in such a country as England, where 

 tuberculosis has been prevalent for unnumbered gener- 

 ations, so many people are inherently immune, is ob- 

 viously related to the fact that immunity cannot be 

 individually acquired against it, for, unlike the case of 

 such a disease as measles, against which immunity may 

 be acquired, inborn immunity is here pushed to its 

 extreme by Natural Selection as the sole means to 

 survival in the presence of the disease. It appears, there- 

 fore, that in the case of diseases the micro-organisms 

 of which do not paralyze the phagocytes with powerful 

 toxins, but engage from the outset in a pli3'sical struggle 

 with them, that immunity cannot be individually ac- 

 quired, and, therefore, that protective inoculation is 

 useless; but in the case of diseases, the less hardy 

 microbes of which, engaging at long range, produce 

 powerful toxins by means of which they paralyze and 

 keep at a distance the phagocytes, immunity may be 

 acquired, if under normal circumstances the reaction, 

 wliereby the phagocytes acquire the power of disregard- 

 ing the toxins, and so of attacking the microbes, is not 

 too great to be accomplished at one effort; and even when, 

 under normal circumstances, the reaction is too great 

 to be accomplished at one effort (as in rabies), it may 

 be brought about by one or more protective inoculations, 

 by means of which the phagocytes are enabled to ac- 

 complish in a shorter or longer series of efforts that 

 which they are unable to accomplish at a single effort. 



