238 ACQUIRED IMMUNITY 



and man. Algerian sheep resist the organism, whereas other 

 breeds of sheep readily succumb. Dogs are practically immune 

 to tuberculosis, while guinea-pigs are killed by the most minute 

 dose of the bacillus. Rats and mice are not susceptible to diph- 

 theria. In none of these cases, however, is the immunity absolute. 

 By altering the circumstances of the animal it is generally possible 

 to render it susceptible to the disease. Thus, by keeping frogs at 

 a raised temperature, it is possible to infect them with anthrax ; and 

 by overtiring animals by excessive work they maybe made suscep- 

 tible to infection with organisms to which they otherwise possess 

 almost complete immunity." x 



399. (cT) As a general rule parasites flourish best in the species of 

 animal which they normally inhabit, doubtless because they have 

 evolved adaptation to that particular habitat ; for this reason 

 human diseases more readily afflict allied species (mammals in 

 general and monkeys in particular) than types less akin, (e) Mic- 

 robic species, if removed from their normal habitat (e.g. from 

 one species of animal to another, or from the living body to non- 

 living media), are apt to alter their degree of virulence. The 

 longer they inhabit a species of animal the more virulent, as a 

 rule, do they grow for that species till a degree of virulence, 

 sufficient to enable the microbes to persist, is reached. Thus the 

 microbes of rabies taken from dogs and passed through a series of 

 rabbits have their virulence immensely exalted for rabbits, though 

 it is said to be lessened for human beings. 2 When transferred to 

 non-living media microbes tend to lose their virulence and become 

 saprophytes. The only rational explanation appears to be that 

 in the former case selection of the microbes that are most virulent 

 to the new type of host, and therefore best able to defend them- 

 selves from his phagocytes, causes progression ; whereas in the 

 latter cessation of selection leads to the loss of the now useless 

 virulence. On the other hand, normally saprophytic organisms 

 may be rendered virulent by placing them in the living body, first 

 in situations where they are least exposed to the phagocytes, and 

 then in situations where they are more exposed. Some organisms, 

 normally saprophytic (e.g. the tetanus bacilli and those which cause 

 putrefaction) are intensely poisonous. Apparently their toxins 

 have been evolved, not through a struggle with phagocytes, but 

 probably with other lowly organisms. 



400. (/) It has been conclusively proved experimentally that if 

 smallpox be passed through a series of calves it becomes cowpox ; 



1 Bosanquet, Serums, Vaccines and Toxines, p. 13. 2 Op. cit., p. 153. 



