THE PRESENT EVOLUTION OF MAN — PHYSICAL 213 



that if the bacteria of any of them be injected into 

 the tissues of an animal — e. g. man — that has not liad 

 the disease,' and is susceptible to it, the phagocytes of 

 the host appear to be poisoned or paralyzed by the 

 toxins of the micro-organisms, and seem incapable of 

 taking up the latter, or, if they do take them up, of 

 digesting and destroying them, being themselves de- 

 stroyed instead. This continues till the death of the 

 host occurs, in cases in which a fatal termination 

 ensues ; but if recovery follows, it is seen that the 

 phagocytes, becoming habituated to the toxins, gradually 

 acquire the power of attacking and overcoming the 

 bacilli, which are thus destroyed, and the organism 

 freed from them. If now a fresh injection of bacilli be 

 made, it is found that they are at once attacked and 

 destroyed by the phagocytes, in other words, it is found 

 that the individual experimented on has become 

 immune. The duration of this state of immunity, as 

 ah'eady explained, is different in different diseases, being 

 short in some diseases — e.g. diphtheria — and usually 

 lifelong in others — e.g. small-pox. In the latter case it 

 is clear that the cells, which acquired the power of 

 attacking the micro-organisms in the presence of their 

 toxins, transmit this power to their descendants, acting 

 therein like simple unicellular organisms ; whereas in 

 the former case — i. e. when the immunity is of short 

 duration — the acquired trait is not transmitted, or is 

 transmitted in a rapidly diminishing extent for a few 

 generations only ; or when the duration of the immunity 

 is very short, it is even lapsed by the very cells which 

 acquired it. 



The above explanation of acquired immunity — that 

 it results from a fit variation, in response to appropriate 

 stimulation, of the cells (phagocytes) belonging to 

 certain lines of the cell-descendants of the conjugated 

 ovum and spermatozoon, whereby they are (and as a 



