And here we must leave them to go to higher vertebrate animals. Although but little is known of fish-life, a very small part even of that little has been given here, and yet we take leave of it with the feeling that its changes and chances are greater than we can ever thoroughly learn. How much pleasure these creatures have in their water-world it would be difficult for us to say; but since we find them playing together, hunting together, sporting in the warm sunshine, and diving and gambolling in the open sea, and sometimes even calling to one another, we cannot but think that life has great charms for them in spite of the many dangers surrounding them. And when, low though they are in the scale of life, we find them (though curiously enough always the fathers) carrying the eggs, building nests for them, and defending the young, we see that even here, in the very beginning of backboned life, we touch the root of true sympathy, the love of parent for child. [Illustration: THE HOME OF THE EARLY AIR BREATHERS] CHAPTER IV. HOW THE BACKBONED ANIMALS PASS FROM WATER-BREATHING TO AIR-BREATHING, AND FIND THEIR WAY OUT UPON THE LAND. So the backboned animals, as fish, have peopled the seas and rivers, and, as the ages have past on, have become more and more fitted to their watery life, little dreaming of another and different life in the world of air above them. And yet in the same pond with the little stickleback, so busy building his nest, there is a creature which could tell him that it is possible to live in both worlds, if only you have the proper machinery to do it with. It is clear that if the backboned animals were ever to live upon land, after they had begun their career in the water, there must have been some among them which learned gradually to give up water-breathing, and to make use of free air; and we shall not have far to seek for creatures which will help us to guess how they managed it. From almost every country pond, or ditch, or swamp, a chorus of voices rises up in the springtime of the year, calling to us to come and learn how Life has taught her children to pass from the water to the air; for it is then that the frogs lay their eggs, and every tadpole which grows up into a frog carries us through the wonderful history of an animal beginning life as a fish with water-breathing gills, and ending it as a four-legged animal with air-breathing lungs.