236 A. A. SCHAEFFER 



sepia, as well as egg yolk, milk, olive oil, and other digestible 

 substances (see p. 398, table 1.), but he says: 



Toutes ces experiences nous montrent avec certitude que 

 l'absorption par les Infusoires des corpuscles suspendus dans 

 l'eau et la formation des vacuoles digestives depend de la nature 

 chimique de la substance dont sont formes ces corpuscles. Sans 

 aucun doute les Infusoires sont capables de distinguer les 

 differentes sortes de nourriture qui peuvent se trouver a leur 

 portee. 



II est interessant de verifier a present si les infusoires peuvent 

 absorber les differentes substances toxiques insolubles dans l'eau. 

 A cet effet, les Infusoires ont ete nourris de differents sels in- 

 solubles de mercure et d'arsenic, et j'ai ete etonne de voir que 

 ces sels sont assez vite absorbes par les infusoires qui arrivent 

 meme a former quelques vacuoles digestives; ces infusoires 

 perissent ensuite bientot. Mais il ne faut pas oublier qui meme 

 l'homme et les animaux superieurs ne sont pas toujours capables 

 de distinguer les substances toxiques, si ces substances sont 

 depourvues de mauvais gout (pp. 401-402). 



It is evident that Metalnikow speaks here of selection by the 

 feeding mechanism, and not of histonic selection, that is by the 

 digestive mechanism of the tissues. 



It is difficult to see how paramecia can be said to distinguish 

 between substances on a chemical basis when indigestible and 

 insoluble substances as well as digestible and soluble, are eaten 

 to about the same extent. 



Choice based upon chemical qualities is qualitative; each 

 chemical substance must be conceived of as acting specifically. 

 According to this conception, when the sense organ is stimulated 

 the quality of the sensation is exactly expressible; it cannot 

 vary excepting as to its amplitude or continuance. There can 

 be no doubt as to the exact nature of the stimulating object. 

 We would therefore expect an animal capable of choosing on 

 this basis, to choose with great precision. We would certainly 

 expect much more precise selection even if the power of selection 

 was very crude, than is shown in Metalnikow's first table. For 

 according to this table glass and sulfur are eaten apparently 

 as readily as milk or starch and eighty per cent as freely as 

 olive oil (table 1, culture C). 



Organismal selection in Paramecium seems therefore to be 



