OURSELVES
This great difficulty was got over, however, in one way and another, and we have already seen how the tides came to the help of life by giving it opportunities to learn how to breathe air when the tide was out, and coming to its rescue with water when the tide was in. And so, after a long time and many failures, the great step was taken; for, while much life remained in the water, and remains there to this day, all the great, high, and wonderful stages in the story of life came after this. Now, when living things came to land and got abundance of oxygen what ad-
little bit warmer, though you cannot notice it. The rule is simply that the amount of heat, or warmth, in any place spreads itself about over everything, so that no one thing is hotter or colder than any other. This is the case with a coldblooded animal, such as a fish. It is just as hot as its surroundings. If it is in very cold water, it is very cold; and if it swims into warm water, it becomes warm too. But now let us contrast with the fishes the case of warm-blooded animals. The fish in your hand is cold, but your hand is warm; your whole body is warm, and
The lung-fish of Central Africa which breathes the free air LIFE ON THE BORDER LINE BETWEEN SHORE AND WATER
vantage exactly did they gain? In the sea there is so little oxygen to be had that the fish cannot use any of it for the purpose of keeping itself warm. And so the fish, like other cold-blooded animals, . behaves, so far as its warmth is concerned, like a piece of stone. If you have a number of different things in a room, and leave them there for some time, each of them will soon be just as hot as the others; if now you bring a tumbler of hot water into the room, it will get cold, and the other things in the room will get a
that is why other things feel cold to your hand. The truth is that air-breathing animals can help themselves to as much oxygen as they please, and after they have taken all they need for necessaries, they can help themselves to more for luxuries. They simply light a fire within themselves and so keep themselves warm. The warmblooded animal is hotter than its surroundings, because it is making quantities of heat inside itself with the help of the large supplies of oxygen it can get from the air. This does not mean that the animal just 327