Part of the answer, at least, is found in nature's hatred and horror of death. She fiercely refuses to have any dead. An empty heaven, a lifeless sea, an uninhabited rock, a dead drop of water, a dying paramecium, are intolerable and impossible. She hastens always to give them life. The succession of strange dwellers to the decaying trees is an instance of her universal and endless effort at making matter live. Such vigilance over the ever-dying is very comforting--and marvelous too. Let any indifferent apple-tree begin to have holes, and the tree-toads, the bluebirds, and the red squirrels move in, to fill the empty trunk with new life and the sapless limbs with fresh fruit. Let any tall, stray oak along the river start to die at the top, and straightway a pair of fish-hawks will load new life upon it. And these other, engrafted lives, like the graft of a greening upon wild wood, yield crops more valuable often, and always more interesting, than come from the native stock. Perhaps there is no more useless fruit or timber grown than that of the swamp-gums (_Nyssa uniflora_) of the Jersey bottoms. But if we value trees according to their capacity for cavities,--the naturalist has a right to such a scale of valuation,--then these gums rank first. The deliberate purpose of a swamp-gum, through its hundred years of life, is to grow as big as possible, that it may hollow out accordingly. They are the natural home-makers of the swamps that border the rivers and creeks in southern New Jersey. What would the coons, the turkey-buzzards, and the owls do without them? The wild bees believe the gums are especially built for them. No white-painted hive, with its disappearing squares, offers half as much safety to these free-booters of the summer seas as the gums, open-hearted, thick-walled, and impregnable. When these trees alone make up the swamp, there is a roomy, empty, echo-y effect among the great gray boles, with their high, horizontal limbs spanned like rafters above, produced by no other trees I know. It is worth a trip across the continent to listen, under a clear autumn moon, to the cry of a coon-dog far away in the empty halls of such a swamp. To get the true effect of a barred owl's hooting, one wants to find the home of a pair in an ancient gum-swamp. I know such a home, along Cohansey Creek, where, the neighboring farmer tells me, he has heard the owls hoot in spring and autumn since he remembers hearing anything. I cannot reach around the butt of the tree that holds the nest. Tapering just a trifle and a little on the lean, it runs up smooth and round for twenty feet, where a big bulge occurs, just above which is the capacious opening to the owls' cave. There was design in the bulge, or foresight in the owls' choice; for that excrescence is the hardest thing to get beyond I ever climbed up to. But it must be mounted, or the queerest pair of little dragons ever hatched will go unseen.