Thus the birds, with their feathery covering and powerful wings, have left their early friends, the reptiles, far far behind. Taught by their many dangers, many experiences, and many joys, they have become warmhearted, quickwitted, timid or bold, ferocious or cunning, deliberate as the rook, or passionate as the falcon, according to the life they have to lead; or, in the sweet tender emotions of the little song-birds, have learned to fill the world with love and brightness and song. If mere enjoyment were all that could be desired in life, where could we expect to find it better than in the light-hearted skylark as she rises in the early summer morning to trill forth her song of joy, or in the happy chuckle of the hen as her little ones gather around her. Yet we cannot but feel that, happy as a bird’s life may be, it still leaves something to be desired; and that, with their small brain and their front limbs entirely employed in flying, they cannot make the highest use of the world. The air they have conquered; and among the woods and forests, over the wide sea, and above the lofty mountains, they lead a busy and happy existence, bringing flying creatures to their highest development, and showing how Life has left no space unfilled with her children. Yet, after all, it is upon the ground, where difficulties are many, conditions varied, and where there is so much to call for contrivance, adaptation, and intelligence, that we must look for the highest types of life; and while we leave the joyous birds with regret, we must go back to the lower forms among the four-footed animals, in order to travel along the line of those that have conquered the earth and prepared the way for man himself. [Illustration: HOME OF THE EARLIEST KNOWN MILK GIVERS] CHAPTER VIII. THE MAMMALIA OR MILK-GIVERS. THE SIMPLEST SUCKLING MOTHER, THE ACTIVE POUCH-BEARERS, AND THE IMPERFECT-TOOTHED ANIMALS. Our backboned animals have now travelled far along the journey of life. The _fish_, in many and varied forms, have taken possession of the seas, lakes, and rivers; the _amphibia_, once large and powerful, now in small and scattered groups, fill the swamps and the debateable ground between earth and water; the _reptiles_, no longer masters of the world, but creepers and skirmishers still holding their own in many places either by agility, strength, or the use of dangerous weapons, swarm in the tropics, and even in colder countries glide rapidly along in the warm sunshine, or hide in nooks and crannies, and sleep the winter away. And the _birds_,--the merry, active, warmhearted birds,--live everywhere, making the forests echo with their song, rising into the heights of the clear atmosphere, till the world lies as a dim panorama below them, crowd the water’s edge with busy fluttering life, and even wander for days and weeks over the pathless ocean, where nothing is to be seen but sky and water.