214 THE PRESENT EVOLUTION OF MAN — PHYSICAL 



consequence the whole organism is) brought into 

 completer harmony with an environment that has been 

 rendered more complex by the presence of pathogenic 

 microbes — is in full accordance with the theory of 

 evolution set forth in the preceding pages. The 

 variations which the phagocytes undergo, when they 

 acquire powers of attacking pathogenic micro-organisms, 

 which were before invulnerable to them, are strictly 

 comparable to the variations undergone by other kinds 

 of cells under appropriate stimulation ; for example, to 

 the variations undergone by skin-cells when subjected 

 to oft-repeated heat or friction, as in the blacksmith's 

 hand, whereby they are able the better to resist heat 

 and friction ; to the variations undergone by muscle- 

 cells, when, as a result of training, they acquire the 

 power of contracting long and vigorously; to the 

 variations which the eye-cells undergo in an educated 

 man, who without inconvenience can for hours scrutinize 

 small print, a proceeding that would render the eyes 

 of an uneducated man dim and painful in a few minutes ; 

 to the variations undergone by the cells of vai-ious 

 other tissues, whereby toleration is established against 

 nicotine, arsenic, &c.; in short, to the variations under- 

 gone by this or that kind of cell in every variation by 

 means of which the organism is brought into harmony 

 with the environment, for any such variation is 

 after all nothing other than a variation, quantitative or 

 qualitative, or both, of certain of its component cells. 

 Moreover, the theory incidentally affords a valid explana- 

 tion of the otherwise inscrutable fact, that acquired 

 immunity against any zymotic disease protects against 

 that disease only and against no other. A 'priori, we 

 might suppose that the acquired powers which enable 

 the phagocytes to attack and overcome the micro- 

 organisms of any disease, would be available, in some 

 degree at least, against the micro-organisms of all 



