90 THE NATURE AND ORIGIN OF LIFE 



By artificial means, such as culture in phenicated bouillon, 

 they obtained anthrax bacteridia which no longer have the 

 slightest virulence for sheep, that is, bacteridia which, if any 

 sheep were inoculated with them, would be assimilated 

 without rendering the sheep diseased. But the organ of 

 resistance to the sheep was not on that account destroyed 

 in these bacteridia. A favourable occasion came to show 

 it. It happens with anthrax bacteridia that the organ of 

 resistance to sheep is defined in the same way as the organ 

 of resistance to the rabbit, guinea-pig or mouse. The least 

 virulent bacteridium for a sheep, if it lives at all, has always 

 been a sufficient organ to struggle at an advantage against 

 a young mouse a day old ; by the time it has killed this 

 first mouse it has become capable of killing a mouse a week 

 old ; and, having killed this mouse a week old in its turn, 

 it will kill an adult mouse, and then a guinea-pig and then 

 a sheep. But you must give it time to recuperate its atro- 

 phied qualities. It is more difficult for a man who has 

 worked with his brain twenty years to become a blacksmith 

 or for a blacksmith to become a professional thinker ; but 

 it is not impossible. The exaggerated development of 

 one organ in an individual does not totally destroy the con- 

 stituent elements of other organs. Between individuals 

 of the same species, even when they are subjected to differ- 

 ent regimens, there are never other than quantitative 

 differences : all have the same parts, only more or less 

 developed in each. We shall see this even in the definition 

 of species in biology. 



Vaccination 



On account of the practical interest of the matter, I 

 note the fact that Pasteur proved, from such phenomena 

 as we have been studying, the general method of vaccination. 



