§4] ACCLIMATIZATION TO EXTREME TEMPERATURES 249 



covering, the water of the body begins to freeze, and in that 

 process, or the subsequent thawings, the protoplasm undergoes 

 a (partly mechanical) change resulting in death. If the body, 

 however, contains no water, freezing cannot kill it. 



Thus the effect of high temperatures is principally chemical, 

 involving the living plasma ; that of low temperatures is prin- 

 cipally mechanical, involving the water of the body. Both 

 raising and lowering the temperature act also as irritants.* 

 Finally, the positions of the maximum and minimum stand in 

 most intimate relation to the inorganic environment of the 

 organism and have been molded to that environment. 



§ 4. AccLiMATizATioisr OF Oeganisms to Extreme 

 Temperatuees 



The phenomena to be discussed in this section fall naturally 

 into two subsections: (1) acclimatization to heat and (2) accli- 

 matization to cold. They will be considered in that order. 



1. Acclimatization to Heat. — Our study of the maximum 

 temperature which organisms reared under ordinary circum- 

 stances can withstand, led us to the conclusion that few active 

 organisms can resist a temperature of over 45°, and for whole 

 groups like Coelenterata, marine MoUusca, and Crustacea, and 

 the fishes, 40° is a point of death. Yet, on the other hand, it 

 has long been known that there are organisms living in certain 

 hot springs in waters of considerably higher temperature. I 

 shall now give in tabular form some cases which I have 

 collected of "organisms living at or above the normally lethal 

 temperature of the species, f 



* Whether sudden change of temperature has an especial effect upon the 

 movement of protoplasm is a disputed question, which has been answered posi- 

 tively by DoTKOCHET ('37, p. 777) and Hofmeister ('67, p. 53) for Nitella, and 

 DE Vkies ('70, p. 894) for root hairs of Hydrocharis, but has since, as a result 

 of careful experiments, been denied by Veltnek ('76, p. 214) for Nitella and 

 other plant cells. 



t It is desirable that accurate data concerning the temperature of organisms 

 in hot springs should be made, and we have, in this countiy, unusually favorable 

 conditions offered for this study, especially in Arkansas, California, and the 

 Yellowstone National Park. It is to be hoped that persons who have had the 

 proper training should, when contemplating a visit to hot springs, provide them-; 



