182 EXPERIMENTAL EVOLUTION LECT. 



while investigating the influence of the different 

 elements which go to make up that complex whole 

 which we call environment, has studied the influence 

 of many chemical substances upon the growth of 

 Aspergillus niger. After having ascertained the 

 exact nature and proportions of the chemical sub- 

 stances which are required to provide for the plant 

 the best suitable medium, he has investigated the 

 influence of some chemicals which do not contribute 

 to the making of that medium. Some of them exert 

 a most unfavourable influence ; thus bichloride of 

 platinum for instance prevents all vital manifestations 

 of Aspergillus ; even when added in the very minute 

 dose of y~oVo-, ** kills the plant. With bichloride of 

 mercury these results are still more striking, as the 

 dose of -5 T oVcro i s enough to kill Aspergillus ; and 

 with nitrate of silver the results are still more 

 surprising : add only i^nhnnr an d death ensues. 



It even happens that when Aspergillus is made to 

 grow in a silver cup, the plant soon dies, because 

 some of the silver dissolves in the liquid medium, and 

 although there is not enough of the metal in the 

 liquid to allow its detection through chemical 

 analysis, the plant detects it immediately, and shows 

 that it feels the influence of the poison. Similar 

 facts are very abundant now, since bacteriology has 

 sprung into existence, and we all know of the con- 



