226 THE PRESENT EVOLUTION OF MAN— PHYSICAL 



their toxins much more rapidly than do the virulent 

 microbes, a thing most unlikely to happen. 



As regards a fourth group of zymotic diseases, we can 

 hardly hope anything from the protective inoculation 

 of attenuated microbes, either before or after infection. 

 Included within this group are tuberculosis and leprosy, 

 which run a prolonged indefinite course in the infected 

 individual, and against the microbes of which the 

 phagocytes never seem to acquire markedly increased 

 powers, behaving in this respect like other kinds of 

 cells when subjected to the action of certain vegetable 

 and mineral poisons — e. g. digitalis and mercury — which 

 they are never able to tolerate the better for training. 

 In leprosy and tuberculosis the toxins, judged by their 

 systemic effects, apparently play a very minor part, at 

 least when compared to the part played by them in 

 other diseases— e. g. diphtheria and anthrax. The 

 battle here is not carried on, so far as has been 

 ascertained by the pathogenic organisms, at long range, 

 but is to all intents and purposes a physical struggle 

 between them and the phagocytes, for at all stages of 

 the disease the former may be seen enclosed in the 

 cell-substance of the latter. In leprosy the microbes 

 are invariably or almost invariably the victors, since 

 never, or very rarely, does a sufferer recover from that 

 disease. In tuberculosis the phagocytes are often 

 victorious, since recovery from this disease is common. 

 But victory does not endow the cells with increased 

 power, as it does in such diseases as small-pox, in which 

 the toxins are abundapt ; stimulation does not cause 

 them here to vary in a fit direction, for he who has 

 once suffered from tuberculosis is as liable as ever to be 

 attacked by the disease. Immunity from tuberculosis 

 and leprosy is therefore entirely or almost entirely of 

 the inborn inherited kind. Some individuals seem 



