138 Lectures on Bacteria. [^ xii. 



we find no morphological or anatomical differences in the con- 

 curring parts in cases which differ furthest from one another, 

 we can only assume that the cause of the difference in behaviour 

 lies in material differences, differences in the chemical be- 

 haviour of the two objects. And since we are dealing on the 

 one hand with portions of an animal which, as far as we can 

 perceive, is not essentially altered in its collective properties, and 

 on the other hand with a Bacillus which has its properties which 

 are in this case the subject of direct observation essentially 

 altered by attenuation, it follows that the changes in the chemi- 

 cal qualities must chiefly be on the side of the Bacillus. Nor is 

 this at all inconsistent with the phenomena of protective inocu- 

 lation, the accustoming the blood-cells, as was said, to the 

 absorption of a succession of Bacilli each more virulent than the 

 preceding one. On the contrary we know that other amoeboid 

 protoplasmic bodies which absorb solid substances, for example 

 the Plasmodia of the Myxomycetes, do become habituated to 

 contact with and probably also to the absorption of bodies with 

 certain chemical qualities, though at first they hastily withdraw 

 from contact with them ; and without further arguments there 

 is good reason for attributing the same power to the blood- 

 cells, because they agree with plasmodia in all other qualities 

 which have any bearing on this point. 



No precise account can at present be given of the nature of 

 the chemical differences between virulent and attenuated Bacilli, 

 and what can be said about it will be mentioned presently. 

 The proximate cause of the attenuation by Pasteur's method 

 is to be sought not in the effect of oxygen but in the 

 heightened temperature; this has been shown convincingly 

 by Chauveau and by Koch and his colleagues, who call 

 attention to the fact that the degree and permanence of the 

 attenuation and also the time required for attaining it are di- 

 rectly dependent, other conditions being the same, on the tem- 

 perature and even on small variations of temperature. We know 

 nothing at present respecting the causes of the attenuations 



