386 ADAPTIVE VARIATIONS. 



After four weeks, the temperature of heat rigor was 

 determined by gradually heating the water containing 

 the tadpoles. Whilst all the tadpoles kept at 15 went 

 into heat rigor at or below 41, those. reared at 25 did 

 not in any case die at a temperature below 43, the 

 average increase of resistance amounting to 3.2. This 

 adaptation to higher temperature gradually disappears 

 on returning the tadpoles to water at ordinary tempera- 

 atures, more than half of the 3.2 increase being lost 

 after keeping them for 17 days at 15. 



Probably the capacity for acclimatisation is present 

 to a greater or less degree in every organism. In some 

 observations carried out at Naples,* I found that the 

 death temperatures of a Medusa (Rliizostoma), a salp 

 (Salpa africana), and of AmpJiioxus were, on an aver- 

 age, respectively 1.3, .6, and 1.5 higher in August 

 than they had been in April, when of course the tem- 

 perature of the sea was several degrees lower. 



The adaptability of the highest organisms to changes 

 of environment does not afford so much support to our 

 thesis viz., that adaptability is a fundamental prop- 

 erty of protoplasm as does that of the lowest organ- 

 isms, because the adaptation is, as a rule, indirect and 

 complex. Still the intrinsic interest of the subject is 

 so great as to warrant a brief reference to it. Almost 

 all of the exact observations deal with acclimatisation 

 to chemical agents, especially the toxins secreted by 

 bacteria. Upon mice Ehrlich * has made some very 

 exact observations on adaptation to a vegetable poison, 

 ricin. The mice were fed on food cakes soaked in 



* J. Pliysiol., xxv. p. 131, 1899. 



t Deutsche med. Wochenschr., 1891, p. 976. 



