220 THE PRESENT EVOLUTION OF MAN — PHYSICAL 



animals were inoculated with a culture from the twelfth 

 day, which before had killed half the animals, there 

 was still a slight febrile disturbance, and none of the 

 inoculated animals died. Virulent anthrax blood might, 

 after a further interval of twelve days, be introduced 

 into animals that had been subjected to the double 

 inoculation, without giving rise to anything more than 

 a slight febrile condition similar to that noticed as 

 resulting from the inoculation of the modified virus. 

 If, however, virulent anthrax blood were introduced 

 into animals, in which only the first protective inocula- 

 tion had been made, — i. e. with material that had been 

 cultivated for twenty days, — a large proportion of the 

 animals died. It was evident, therefore, that it was 

 absolutely necessary to use both a first and a second 

 vaccine if this protection was to be complete. This 

 attenuation was not confined to the generations of the 

 bacilli that were directly acted upon. Tf the temperature 

 were lowered to about 35° C, vegetative activity was 

 immediately set up, rods in enormous numbers were 

 formed, and eventually spores might be observed in 

 these rods. Now comes the interesting fact : the 

 attenuated properties of the original bacilli were handed 

 on to the spores ; these spores might be kept in a latent 

 condition for a considerable length of time, and on 

 being introduced into media suitable for their growth, 

 they sprouted out, not into virulent anthrax bacilli, but 

 into modified anthrax bacilli, so that the conservation of 

 the vaccine (on silk threads) became a comparatively 

 easy matter." — Bacteria and their Products, pp. 372-3. 



Judged then by the most delicate of our tests, the 

 effects of its toxins on living creatures, the anthrax 

 bacillus is not only modifiable by a changed environ- 

 ment, but its acquired traits are transmissible to its 

 descendants. We must admit this, or hold as the only 

 alternative, that the progressive modification of the 

 microbe in the changed environment is due to a process 

 of natural selection among them, causing evolution in 



