320 PASTEUR: THE HISTORY OF A MIND 



When it is the host which succumbs, the microbe seems 

 to emerge more inured to the struggle, capable of secret- 

 ing in greater abundance the products which have ren- 

 dered it victorious. We explain this fact by saying that it 

 has become more virulent, and a good way of increasing 

 its virulence is to make it pass through species, which 

 without being absolutely immune, can resist it a long 

 time and enable it to acquire a new vigor. That is what 

 we did when we rendered the anthrax bacteridium more 

 virulent by making it pass through species more and more 

 resistant to its action. 



On the other hand, when it is the microbe which suc- 

 cumbs in the struggle, the leucocytes in their turn issue 

 from the conflict stronger, more sensitive to the chem- 

 iotaxis of the microbes which they have killed, and more 

 accustomed to their toxines, and the animal consequently 

 has a power of resistance, an immunity, which it did not 

 formerly possess. 



It is not necessary to enter into details to see that we 

 have here a conception which lends itself in a remarkable 

 manner to the interpretation of all the very curious facts 

 discovered by Pasteur. I add that this interpretation 

 is not purely theoretical. It is sufficient to read, in the 

 Annales de VInstitut Pasteur, the numerous works ac- 

 cumulated on this subject by M. Metchnikoff and his 

 pupils, to be convinced that we are face to face not only 

 with a captivating theory, but with a theory true to its 

 smallest details, and in all respects fruitful. 



In resume, the resistance of each living being with 

 respect to a microbial inoculation is at the same time 

 a question of species, a question of individuals, a question 

 of place and of time, a question of quantity of inoculating 

 material, and also a question of temperature, for a low- 

 ering of temperature can diminish the activity of the 

 leucocytes and increase that of the bacillus, as in case 



