192 



MICROBES AND TOXINS 



side the body in the test-tube. It must be carefully kept in 

 view that all the most plausible arguments in its favour are 

 supplied by " in vitro " experiments. It is a laudable 

 tendency to attempt to reduce biological phenomena to a 

 mechanism reproducible at will, but it must not be allowed to 

 distort the facts of nature. The study of immunity is, above 



all, the study of an infected 

 body defending itself. We 

 know neither the nature 

 nor the composition of 

 the substances concerned, 

 albuminoid or otherwise : 

 we are not even always sure 

 that the substances postu- 

 lated really exist- Too 

 often we yield to the ten- 

 dency to describe as sub- 

 stances what we only 

 observe as functions ; these 

 functions have been sym- 

 bolised by names and by 

 signs, and some have even 

 come to see in them actual things and things with an actual 

 shape. It cannot be denied that at present, while we are 

 still awaiting the advances so generally longed for, vitalism 

 (in the sense in which certain critics of the phagocytic theory 

 employ the word) represents the most realistic conception. 



Metchnikoff has therefore never ceased to recall and empha- 

 size the differences which separate the corresponding (one 

 cannot say " the same ") phenomena " in vivo " and " in vitro " : 

 not that he disputes the importance of the latter, but to empha- 

 size the necessity of associating them always with the phenomena 

 in the living animal. Experiment ought always to deal as much 

 as possible with the living creature itself. The cells and the 

 body-fluids are hardly to be treated as substances capable of 

 preservation in bottles, and this fact we will have occasion to 

 recall more than once. 



Fig. 65. — Phagocytes taking up spores 

 of the tetanus bacillus (heated). 



