82 PASTEUE: THE HISTORY OF A MIND 



would open a new world, the world of the bacteria, a 

 world still more active and more densely populated than 

 the world of the yeasts. 



There was, in this same Note which we are analysing, 

 a fact much more important than the animal or plant 

 character of the butyric vibrio: namely, that this organ- 

 ism lives in the absence of the oxygen of the air and even 

 fears its contact. Pasteur has often related how this 

 fact leaped, so to speak, into his field of vision. In 

 examining these liquids, he would take a drop, place it on 

 'the slide, cttver it quickly with a cover glass, which 

 spread it out in a flat layer, and put the preparation 

 under his microscope. But on examining, with the 

 care which he applied to everything, one of these little 

 flattened drops of liquid undergoing butyric fermenta- 

 tion, he was astonished to see that on the margins of the 

 little drop, wherever it was in contact with the air, the 

 bacteria had become non-motile and inert, although they 

 continued to move with agility in the central portions. 

 This was a spectacle quite the reverse of that which he 

 had had occasion to observe often in the case of the 

 animalcules of the infusions. Especially when we ex- 

 amine, under the microscope, those from the surface of 

 infusions, they voluntarily leave the central portions of 

 the drop to approach the margin, the only place where 

 there is enough oxygen for all. In the presence of this 

 observation, Pasteur asked himself immediately: is it 

 true that these vibrios are trying to escape from the 

 oxygen? An experiment along this line was easy to 

 make. By passing a current of air through a flask in 

 which the butyric fermentation was going on, the fermen- 

 tation was retarded or arrested, and behold a new idea 

 was introduced into science, the idea of anaerobic life 

 as opposed to aerobic life which was believed to be that 

 of all the animals of creation. We shall see how Pasteur 



