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 inoculation 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. If 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 environment, 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
