228 THE PRESENT EVOLUTION OP MAN— PHYSICAL 



Against these diseases protective inoculations of attenu- 

 ated microbes or of their toxins are therefore useful. 

 But owing to the circumstance that the duration of 

 acquired immunity is different in different diseases, 

 from practically life-long immunity in some diseases 

 — e.g. scarlatina and small-pox — to immunity which 

 endures for but a few days — e.g. relapsing fever — the 

 degree of utility that may be hoped for from protective 

 inoculations varies. In small-pox and kindred diseases, 

 in which the duration of immunity is long, it may be 

 most useful ; in relapsing fever and kindred diseases, 

 in which the duration of immunity is short, it is much 

 less useful, except, indeed, in the cases of such dangerous 

 diseases as diphtheria, which saturate the host with 

 their toxins slowly, when inoculation by attenuated 

 microbes or their toxins helps the infected individual to 

 tide over a dangerous crisis, and enables his phagocytes 

 by a series of efforts to accomplish a reaction which they 

 may be unable to perform at a single effort. The quick- 

 ness and completeness with which the immunity is pro- 

 duced, though not the duration of it, seem in direct 

 proportion to the quickness with which the toxins are 

 produced, and to their degree of virulence. To put it 

 in another way : Natural Selection has so dealt with 

 the micro-organisms of some diseases — e.g. small-pox, 

 measles, &c. — that there has been an evolution in them 

 of the power of producing toxins as their chief means of 

 combating the phagocytes; in which case the phago- 

 cytes must react quickly or perish; if they do react 

 quickly they find the feeble microbes an easy prey. But 

 as regards diseases in which the toxins are neither quickly 

 produced nor virulent — e.g. tuberculosis — Natural Selec- 

 tion has so dealt with the pathogenic micro-organisms 

 that there has been an evolution in them of what may be 

 called personal vigour as their chief means of combating 

 the phagocytes. In such diseases the struggle between 



