It is, perhaps, natural that we should shrink from cold-blooded creatures, especially as they _seem_ to show very little affection. Yet lizards, tortoises, and snakes can all be made to know and care for those who are kind to them; while, as we have seen, the fierce crocodile watches over and feeds her young, and the python coils herself over her eggs, and will take no food till they are hatched. Moreover, we can scarcely look at the quaint shell-covered tortoise, or examine the heavily-mailed coat of the alligator, or the poison-fangs of the snake, without admiring the curious devices by which these animals have managed to survive in a world which once belonged to their ancestors, before our present swarm of warm-blooded animals multiplied and took possession of their kingdom. [Illustration: THE EARLIEST KNOWN WATERBIRDS] CHAPTER VI. THE FEATHERED CONQUERORS OF THE AIR. PART I.--THEIR WANDERINGS OVER SEA AND MARSH, DESERT AND PLAIN. It is a warm sunny day in early spring, one of those few bright days which sometimes burst upon us in April, just after the swallows have come back to us, searching out their old nooks under the eaves, or their old corners in the chimneys, to build their new nests. There they are, clinging with their sharp claws to the edge of the cottage thatch, while the impudent little sparrow, which has remained hopping about all the winter long, chirrups at them from a neighbouring apple-tree. Upon the grass-plot near a blackbird is pecking at a worm, and from the wood beyond a thrush trills out his clear and mellow song, accompanied from time to time by the distant cry of the cuckoo calling to his mate. For it is the love-time of the birds; and as we watch them flying merrily hither and thither in the bright sunshine, we ask ourselves whether we must not have made a great leap on leaving the cold-blooded snakes and tortoises, since now we find ourselves among such merry, warm-hearted, passionate little beings, with their beautiful feathery plumage, their light rapid flight, their love for each other, their skill in nest-building, and their patient care for their little ones. And, indeed, we have come into quite a new life, for now we are going to wander among the conquerors of the air, who have learned to rise far beyond our solid ground, and to soar, like the lark, into the clouds, or, like the eagle, to sail over the topmost crags of the mountains, there to build his solitary eyrie. Even the little sparrow, which flits about by the roadside, can laugh at us with his impudent little chirp, as he flies up out of reach to the topmost branch of a tree. And yet a glance at his skeleton will show us that he has the same framework as a reptile, only it is altered to suit his mode of life. [Illustration: Fig. 32. The Sparrow. With wings raised, as in the skeleton on next page.]