LIFE OF ELIE METCHNIKOFF 191 



to allow of a local treatment which might succeed 

 in being curative as it had time to act before the 

 microbes had passed into the general circulation of 

 the organism. This supposition was proved to be 

 correct by a series of experiments on monkeys, and, 

 in 1906, a young doctor, M. Maisonneuve, inoculated 

 himself with syphilis and applied the treatment with 

 a perfectly satisfactory result. 



It might have been thought that this simple, safe, 

 and innocuous method would at once come into prac- 

 tice, but it was not so. Between opposition on the 

 one hand, and carelessness of the subjects themselves 

 on the other, this useful discovery remained for a long 

 time without being utilised. All the above results were 

 obtained through experiments on anthropoid apes, 

 and the study of syphilis, until then purely clinical, 

 entered at last into the field of experimental science. 



Researches upon syphilis were but an interlude ; 

 Metchnikoff, returning to his principal work, resumed 

 the study of senility and of the intestinal flora. 

 During many years he applied himself to researches 

 concerning the part played by the latter within the 

 organism. 



He was able to confirm the deductions expounded 

 in his Etudes sur la nature humaine, and in 1907 he 

 published a new work, Essais optimistes, in which he 

 developed the same ideas, amplified by the results 

 of his new researches, and answering the criticisms 

 excited by his first book. 



In the Essais optimistes he studied first of all the 

 phenomena of old age in the different grades of the 

 scale of living beings, of which he compared the life 

 duration. He concluded that there was an indubit- 



