572 J- Frank Daniel 



single cell is fundamental, the following series of experiments have 

 to do with the adjustment and immunization of different types 

 of lower organisms to various kinds of substances. 



The fact that the same species of protozoa may be found under 

 widely differing environmental conditions — in fresh water or in 

 alkalin lakes — suggests at once that what protoplasm is in kind 

 depends much upon the experiences through which it has passed. 

 If this be true the toleration of different environmental conditions 

 just mentioned would probably be due to the gradual adjustment 

 of the organisms to slight but constant changes in the surroundings. 

 By such a process different members of a single race might finally 

 come to live and prosper as separate types under conditions so 

 widely different as to be mutually destructive. 



That such extremes of adjustment are possible may be demon- 

 strated in the laboratory. 



Various investigators by subjecting organisms for a time to 

 heat or cold have rendered them immune to great extremes of tem- 

 perature.^ Thus Dallinger^ in a continuous series of experiments 

 lasting through a period of several years so increased the resistance 

 of various Flagellates to heat as to raise the killing point from 23° 

 C. to the extreme of 70°, so that the organisms lived at a tempera- 

 ture 47° higher than that which formerly destroyed them. 



Experiment with poisonous chemicals likewise shows that sub- 

 stances which are extremely destructive to living protoplasm may 

 be rendered less injurious if the animal is subjected gradually to 

 increasing concentrations. Davenport and NeaP have shown 

 that the unicellular organism Stentor coeruleus after being kept 

 for a few days in a weak concentration of corrosive sublimate, is 

 able to survive a stronger solution several times as long as can a 

 control animal of the same series. 



A beautiful case of adaptation has been shown by Hafkine.* 

 In his experiments a drop of natural water containing different 



* For a reference to the literature, see Davenport and Castle. 1895, Arch. f. Entw. mechan., II Band, 

 2 Heft, pp. 227-249. 



2 Dallinger, W. H., 1880, Jour. Royal Mic. Soc, III, pp. 1-16. 



^ Davenport, C. B., and Neal, H. V., 1896, Arch. f. Entw. mechan., II Band, 4 Heft, pp. 564-583. 



* Hafkine, W. M., 1890, Ann. de'l institut Pasteur, iv, pp. 363-379. 



