a changed direction by the accumulation of inborn variations of a kind that in the former environment did not conduce to survival. It is highly improbable, however, that the latter theory correctly explains the facts, (1) because the time in which the modification is obtained is apparently too short to admit of its arising as a result of the survival of the fittest, and (2) because we have no reason to doubt that acquired traits are transmissible in the microbes, since they are all very low unicellular organisms, each one of which, since it is capable of continuing the race, is to be regarded as a germ cell on which the environment acts directly. The point, though of great interest, is, however, of theoretical importance only. The fact remains, that the bacilli of anthrax, whether by the transmission of acquired variations or by the accumulation of inborn variations, or both, are easily to be modified by changing the environment; and this has been proved true, not only of the bacilli of anthrax, but of the microbes of numerous other diseases as well, including the microbes of rabies, as was shown by the experiments above cited, in which Pasteur demonstrated that while these microbes, when cultivated through a succession of dogs, are fatal to the dog and other susceptible animals, when cultivated through a succession of monkeys induce only a transitory illness in dogs, and when cultivated in a slowly drying cord do not prove fatal to human beings.
We are now in a position to attack the difficulty we encountered a few pages back—to explain how it is that in a man bitten by a rabid dog, hydrophobia is not intensified, but cured by a treatment which begins with the injection of material from the desiccated cord of an animal dead of that disease, proceeds with the injection of fresher and fresher infective material, and ends with the injection of the most virulent material.
