X04 NATURAL SCIENCE. Feb.. 



hand, it is not a normal constituent, how could it ever be present 

 without injury ? And, under the latter supposition, a person in 

 perfectly normal health would receive no benefit from inoculation, 

 since there would be nothing for the modified microbe to remove. 



Nor has it been explained how the microbe of the modified 

 cultivation could multiply in the body without producing the same 

 inconvenience as the original form, supposing the microbe is, in itself, 

 the cause of the disease. 



If, as has been suggested, the malady is caused by the mere 

 number of the microbes clogging the veins, obstructing the circulation, 

 and taking matter from the blood, then the modified form would 

 accomplish this as readily as the other. 



But passages might be quoted which seem to imply Pasteur's 

 belief that the " attenuation " of the virus is due to the loss of power 

 to multiply in the animal body. The following is from the English 

 translation of the life of Pasteur : — 



" After having reduced the microbes of fowl, cholera, and splenic 

 fever to all degrees of virulence, and brought them to a point where 

 they could no longer multiply in the bodies of animals inoculated 

 with them, and fixed them in media appropriate to their life, Pasteur 

 asked himself whether it would not be possible to restore to these 

 attenuated microbes — weakened to such a degree as to have lost all 

 virulence — a deadly virulence, and to render them again capable of 

 living and multiplying in the bodies of animals." 5 



Enfeebled in such a way, the microbes could scarcely exhaust 

 the blood of that which would have supported a sufficient number of 

 the unmodified microbe to produce disease. 



Again, in the following passage it is distinctly asserted that the 

 lack of virulence is due to the loss of power of living in the animal 

 body : — 



" The extraordinary fact is then established that the virulence 

 may be entirely gone while yet the microbe lives. The cultures offer 

 the spectacle of a microbe indefinitely cultivable, yet, on the other 

 hand, incapable of living in the bodies of fowls, and, in consequence, 

 deprived of virulence."^ 



In the modification of the splenic fever microbe the cultiva- 

 tions gradually lose their power of increasing in the artificial medium, 

 and finally refuse to multiply any further ; the last sowing does not 

 produce a crop. It is one or more of the cultivations preceding the 

 last which are used as vaccine. 



Can this microbe, which has almost lost the power of reproduc- 

 tion, be supposed to be able to exhaust the blood of that which would 

 support an abundant crop of the unmodified form ? And if this Scylla 

 of lack of reproductive power be hypothetically avoided, the attempted 

 explanation splits on the Charybdis of abundant reproduction 

 producing all the evil effects of the unmodified virus. 

 5 0/. cit., p. 246. '■ Op. cit., p. 226. 



