THE PRESENT EVOLUTION OF MAN — PHYSICAL 221 



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 trans- 

 missible 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. 



