168 Heat and Organization. 



might not have germinated and grown above the temperature 

 of his experiments. 



From these facts it will appear that both high and low 

 kinds of plants will stand what might be deemed an excess of 

 temperature in the direction of heat ; but few plants, except 

 those of low organization, will survive extreme cold. Within 

 the polar circle, the nearest approach to a tree is a minute 

 willow, six inches high ; but the short summer is gladdened 

 with rushes, and a few flowers such as Potentilla, Ragged 

 Robin, and Bachelors' Buttons. The greatest cold is sustained 

 by Cryptogams, and the yeast plant will survive 76° below 

 Zero.* 



The animal world can resist great extremes of cold if sup- 

 plied with suitable food. The fur of the Polar bear keeps in 

 the heat which is generated by respiration and other functions 

 of the body, in which a slow combustion occurs. In such 

 cases the creature is warm, although the atmosphere he lives in 

 is cold. 



The effect of heat is to accelerate 'the processes of life, and 

 if a cold-blooded animal, such as a frog or a serpent, is kept 

 very warm, its activity is i stimulated, and its life brought to a 

 speedier close. Some creatures can stand a complete sus- 

 pension of vital action without being killed. Thus in one 

 of Captain Ross's voyages certain caterpillars were frozen, so 

 that they chinked like ice when thrown into a tumbler, and re- 

 sumed their activity when thawed. Even animals so high as 

 certain fishes may be frozen so as to become quite brittle with- 

 out life being destroyed, and the mud-fish of Africa may be 

 dried to a chip without being killed. Warm-blooded animals 

 are more limited in their powers of endurance, and an arrest of 

 vitality that would not hurt a reptile, is usually fatal to them. 



" The highest limit of temperature compatible with the life 

 of fishes has not been certainly ascertained, and it appears 

 probable that there are considerable variations in this respect 

 amongst different species. Thus it is certain that there are 

 some which are killed by immersion in water at 104°, while it 

 is also certain that others can not only exist, but can find a con- 

 genial habitation in water of 113°, and even of 120°,and examples 

 of the existence of fishes in thermal springs of a much higher 

 temperature have been put on record. Various fresh water 

 mollusks have been found in thermal springs, the heat of vrfaicb 

 is from 100° to 14-5°. Rotifers and other animalcules have been 

 met with in water at 112°. Larvae of tipulse have been found 

 in hot springs of 205V ; f After this we cannot be surprised 



* Carpenter's Manual of Physiology, p. 69. 

 t Ibid, p. 86. 



