THE PRESENT EVOLUTION OF MAN— PHYSICAL 219 



symptoms, and practically in the same time. When 

 inoculated from the dog to tlie monkey, however, the 

 virus becomes less virulent ; it is said to be attenuated 

 or weakened, the attenuation becoming more and more 

 marked in successive inoculations from monkey to 

 monkey ; the course of the disease becomes longer and 

 longer, until eventually there may come a time at 

 which the virus, when introduced under the skin or 

 into the cranial cavity, is not sufficiently active to cause 

 the death of this species. If this attenuated fluid be 

 now inoculated into a rabbit, a dog, or a guinea-pig, it 

 still remains comparatively weak for a time, through 

 successive inoculations on these animals — i. e. at first it 

 does not kill, then it kills, but only after a considerable 

 time ; but gradually the virulence returns, until at last 

 it reaches its original level of malignancy, whilst if the 

 successive inoculations are made in rabbits with primary 

 fluid from either the dog or the monkey, the virulence 

 may become so exalted that it is considerably greater 

 even than that of the virus taken from the street dog, 

 which at one time was supposed to be the most virulent 

 form except that of hydrophobic wolves, which has 

 always been known to be specially fatal ; the virulence 

 is doubled as the inoculation period is reduced to one- 

 half." — Woodhead, Bacteria and their Products, p. 320. 



"Pasteur '. . . subjected the bacilli" (of anthrax in 

 a pure culture, i. c. in an artificial nutrient medium free 

 from other kinds of micro-organisms) " to a temperature 

 of from 42° to 43° C, ; these were found to have lost all 

 their vitality at the end of about six weeks, this loss of 

 vitality during the six weeks going on progressively in 

 proportion to the rise of temperature. It is stated that 

 at the outset the pure culture had all the virulence of 

 anthrax blood ; whilst only half of the sheep inoculated 

 with the culture that had been heated for ten days 

 succumbed to anthrax. On the twenty-fourth day of 

 heating, the culture, when inoculated, although giving 

 rise to mild febrile disturbance, did not cause the death 

 of a single animal. It was found, too, that if now, 

 twelve days after the first inoculation, these surviving 



