INBORN AND ACQUIRED IMMUNITY 255 



due to one of two very different causes ; a very high resisting power 

 on the part of the individual attacked, or to a low attacking power 

 on the part of the microbes. The high resisting power is due to 

 stringency of selection affecting the species possessing it ; the low 

 attacking power is due to lack of selection amongst the microbes. 

 Thus human races that have been much afflicted by tuberculosis 

 are, as compared to other human races, especially resistant. Dogs 

 on the other hand are almost quite immune. Since we have no 

 reason to suppose that these animals have ever suffered from the 

 disease, we are driven to the conclusion that the environment they 

 offer is unsuitable to the tubercle bacilli. It is one to which their 

 evolution has not adapted them. It must be noted, however, that 

 an evolution, which fits a race for a particular environment, does 

 not necessarily unfit it for existence in another, and, within limits, 

 a different one. Thus many European plants and animals, in- 

 cluding man himself, flourish exceedingly in New Zealand and 

 elsewhere. So also the bacillus tuberculosis, which, so far as we 

 know, has never afflicted wild guinea pigs, is very deadly to them 

 in captivity. In rare cases inborn immunity appears to be a 

 correlate of other and apparently quite unrelated characters. 

 Thus, unlike coloured varieties, white rats are immune to anthrax. 

 426. The main purpose of the present chapter is to establish 

 a clear distinction between inborn and acquired immunity 

 between the kind of immunity which arises under the stimulus of 

 nutrition and that kind which develops through individual experi- 

 ence of disease. To achieve this object we have been forced to 

 discuss the rationale of acquired immunity. The two kinds are 

 constantly confused. Thus in medical discussions, one has only 

 to maintain that parental diseases and acquired immunity are not 

 * hereditary, 3 and some one is sure to retort that in both instances 

 the contrary is constantly observed in syphilis. Many doctors are 

 still convinced that ' common sense ' demands a belief in the 

 Lamarckian doctrine. Certainly nearly all doctors are convinced 

 on grounds of common sense that ill-health tends to so affect the 

 germ-plasm that, in consequence, offspring of unhealthy people 

 are commonly born degenerate. In other words they believe that 

 parental disease is a frequent cause of variations in offspring. We 

 have already discussed this problem somewhat in detail, but it is 

 of great practical as well as theoretical importance and will repay 

 further study. The ground is now cleared of possible misconcep- 

 tions and we may proceed to discuss it in comfort. 



