218 MICROBES AND TOXINS 



to apply physico-chemical laws to biological phenomena in 

 spite of the variability of living creatures and of their products. 

 It is impossible for biologists to refrain from seeking quanti- 

 tative laws and from applying physico-chemical laws to 

 immunity phenomena. Quantitative results have been ob- 

 tained in the study of diastases, and Ehrlich has discovered 

 facts of the greatest interest by means of his experiments 

 in vitro on titrations and measuring. It is only necessary to 

 agree upon the conventions necessary (in physics itself these 

 cannot be dispensed with) and not to employ such unjustifiable 

 expressions as " gukrison in vitro " (in vitro cure). 



It is always possible to return to the biological point of 

 view when this becomes necessary, as Ehrlich himself did, 

 when considering Weigert's ideas on the regeneration of 

 protoplasm. 



THE PHYSICAL POINT OF VIEW. 



Bordet rejected Ehrlich's system, and compared the " anti- 

 gens + anti-bodies " reactions to phenomena of absorption and 

 molecular adhesion, even before the closer comparison with 

 colloidal reactions had been arrived at. 



He does not only complain that the side-chain theory is too 

 complicated ; he criticises its whole disposition. 



Immunity is a problem not yet ripe ; and the solution will 

 probably come from a quarter quite unexpected. There 

 are enormous gaps in our knowledge. Why, then, make 

 adventurous generalizations when the biological facts are far 

 from permitting this ? Every theory that can be constructed 

 must base itself for the moment on facts not yet demonstrated. 

 Let us keep close to the experiments, and be content to 

 advance step by step. Ehrlich's theory is dangerous, in that 

 it offers too readily conceptions which have the appearance of 

 explanations, and which, therefore, are apt to dull the appetite 

 for research. " For my part," adds Bordet, " I have been 

 unwilling to construct a theory ; I do not adduce any general 

 conceptions ; the hypotheses which I have proposed are 



