206 THE PRESENT EVOLUTION OF MAN — PHYSICAL 



to one of permanent disablement, and would place the 

 individual so attacked in the ranks of the unfit, and there- 

 fore it is evident that this power of acquiring immunity, 

 of varying in a fit direction in response to appropriate 

 stimulation, like all other powers of acquiring fit traits, 

 whereby the individual is brought into harmony with a 

 more complex environment, has resulted from Natural 

 Selection. This power is, in fact, a short cut, by means 

 of which is brought about in every individual that takes 

 the disease, or at least in all such individuals as have 

 not reverted to an ancestral condition, in which the 

 power was not developed, or was less developed, a con- 

 dition of complete or nearly complete immunity; a 

 condition of immunity more complete than could 

 possibly have been attained by the survival of the 

 inherently immune alone, if only for the reason already 

 stated, that in the latter case, whenever the immunity 

 approached perfection, the consequent relaxation of the 

 stringency of Disease Selection would cause a racial 

 lapsing back to an ancestral condition of less immunity. 

 It should be noted here, that in all microbic diseases, 

 which run a definite course of limited duration in the 

 individual, immunity, for a greater or lesser length of 

 time after a first attack, must necessarily follow the 

 cessation of that attack, for, since the cessation of the 

 disease is due to the death or departure of the organ- 

 isms which cause it, that which causes' the death or 

 departure of the organisms will for a greater or lesser 

 length of time operate against and cause the death or 

 departure of any fresh organisms of the same kind which 

 may happen to invade the body. Hence it is that who- 

 ever has had an attack of small-pox or diphtheria, is 

 for a greater or lesser length of time immune to those 

 diseases. 



The facts that many zymotic diseases run a definite 

 course, which is limited in point of time, and that one 



