222 THE PRESENT EVOLUTION OF MAN— PHYSICAL 



We have seen that the bacilli of anthrax when exposed 

 to a temperature of from 42° to 43° C, gradually lose 

 their virulence and at length cease to cause death ; in 

 other words, the toxins which they manufacture gradually 

 become less and less poisonous, till they become so little 

 poisonous that the phagocytes of the infected animal 

 are able to vary fitly in response to appropriate stimu- 

 lation, that from the toiins, and become capable of 

 destroying the invading bacilli in spite of the presence 

 of those toxins. We have seen also that if we proceed 

 step by step, first injecting into the tissues of a suscept- 

 ible animal bacilli of little virulence, then after an 

 interval bacilli of greater virulence, we are able at 

 length to confer immunity against the bacilli of the 

 greatest virulence — of virulence so great that they would 

 have infallibly destroyed the animal experimented on 

 had it not been protected by the previous inoculations. 

 The rationale of the process is then clear; the stimula- 

 tion from the feeble toxins of the first injected bacilh 

 induce such a reaction, such a fit variation in the 

 phagocytes of the host, that they are thereby placed in 

 a position of advantage, owing to which they are able 

 to vary fitly in response to stimulation from the stronger 

 toxins of the second injection, which, but for their 

 position of advantage, would destroy or paralyze them. 

 In like manner, from this second position of advantage 

 they are again able to vary fitly still farther in response 

 to the stimulation of the toxins of the third and most 

 virulent injection. So skin-cells when subjected to 

 long-continued heat or friction vary in response in a 

 fit direction, till they are able to withstand degrees of 

 heat and friction which would be fatal to them had 

 they not attained a position of advantage. So cells of 

 other kinds by an analogous process of slow training 

 are able to withstand nicotine, opium, alcohol, arsenic, 

 &c., when present in their nutrient fluids in a degree 



