THE PRIMARY CONDITIONS OF ANIMAL LIFE 109 



for some time and then gradually thawed them out un- 

 hurt. Only very hardy kinds adapted to the cold would, 

 however, survive such treatment. There is no doubt that 

 every part of the body, all of the living substance, of these 

 fish was frozen, for specimens at this temperature could be 

 broken and pounded up into fine ice powder. But a tem- 

 perature of 20 C. killed the fish. Frogs lived after being 

 kept at a temperature of 28 C., centipeds at 50 C.,and 

 certain snails endured a temperature of 120 C. without 

 dying. At the other extreme, instances are known of ani- 

 mals living in water (hot springs or water gradually heated 

 with the organisms in it) of a temperature as high as 50 C. 

 Experiments with Amoeba show that these simplest animals 

 contract and cease active motion at 35 C., but are not killed 

 until a temperature of 40 to 45 C. is reached. The little 

 fish called blob or miller's thumb ( Coitus ictalops] has been 

 seen lying boiled in the bottom of the hot springs in the 

 Yellowstone Park ; but it must have entered these springs 

 through streams of a temperature little below the boiling 

 point. 



The pressure or weight of the atmosphere on the sur- 

 face of the earth is nearly fifteen pounds on each square 

 inch. This pressure is exerted equally in all directions, so 

 that an object on the earth's surface sustains a pressure on 

 each square inch of its surface exposed to the air of fifteen 

 pounds. Thus all animals living on the earth's surface or 

 near it live under this pressure, and know no other condi- 

 tion. For this reason they do not notice it. The animals 

 that live in water, however, sustain a much greater pres- 

 sure, this pressure increasing with the depth. Certain 

 ocean fishes live habitually at great depths, as two to five 

 miles, where the pressure is equivalent to that of many 

 hundred atmospheres. If these fishes are brought to the 

 surface their eyes bulge out fearfully, being pushed out 

 through reduced expansion ; their scales fall off because of 

 the great expansion of the skin, and the stomach is pushed 



