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 Amebe 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 (Cottws 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, se 
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 
