ADAPTIVE VARIATIONS. 381 



allies. All such instances as these * are probably due 

 to gradual acclimatisation, accompanied by a variable 

 amount of selective destruction of the less adaptable 

 organisms. 



Other observations on acclimatisation among Pro- 

 tozoa have been made by Davenport and Neal.f The 

 acclimatisation of Stentor cceruleus to weak corrosive 

 sublimate solution was tested. Stentors kept for two 

 days in .00005 per cent, solution were found, on immer- 

 sion in .001 per cent, solution of sublimate, to be killed 

 off after (on an average) 304 seconds' exposure. Sten- 

 tors kept in pure water, on the other hand, were killed 

 after only 83 seconds 7 immersion. Similar results were 

 obtained in other experiments, it appearing that within 

 certain limits the resistance period varied directly with 

 the strength of the solution in which the protoplasm 

 had been cultivated. If, however, the culture solution 

 were too strong, (above .0001 per cent.), the organism 

 became so weakened that it was less resistant to the 

 killing solution than those reared in pure water. As 

 no deaths occurred in the culture solutions, the adapta- 

 tion must have been a direct one, and in no way de- 

 pendent on natural selection. 



Stentor can also become acclimatised to mechanical 

 stimuli, for Castle $ has observed a colony of Stentors 

 in an aquarium being constantly struck by Tubifex 

 moving backwards and forwards, and yet showing no 

 contraction as they usually do when struck. 



*For a detailed account see a paper by Davenport and Castle, 

 Arch, f . Entwick. d. Organismen, Bd. ii. p. 227, 1895. 

 t Arch, f . Entwick. d. Organismen, Bd. ii. p. 564, 1896. 

 \ Vide Davenport's " Experimental Morphology," p. 109. 



