It must not be concluded that of all these birds perfect specimens have been found. In some cases eggs, feathers, nests, footprints, or stray bones have been the only traces. The extinct bird frequently has had to be reconstructed from small portions of its body. Each succeeding age of the last world-building period has produced new birds. In France the strata from the first age of this period have yielded many species. The ancient beds of lakes have been especially rich in bird remains, as lying in the undisturbed ooze, the fragile bodies were less exposed to destruction. Very many of these birds of ancient days represented forms now living,--as barn-yard fowls, storks, woodcock, crows, and falcons. Possibly the birds were not more numerous in the Miocene than in the succeeding age, but its numerous lake beds, and other circumstances may have been more favorable to the preservation of the remains. A great many fossil birds and remains of birds have been found in the caves of the succeeding eras. Many varieties of birds are still given to inhabiting caves, or any quiet and shady place. No doubt “the temple haunting martlet” haunted caves before temples were built, and swallows that now multiply in chimneys and eaves must have gone for shelter to the crevices of the rocks in ages before men displayed architectural talent. Thus the caves of the ancient world had numerous bird inhabitants, while the captured bodies of birds were no doubt carried to their cave-dens by carnivorous animals. The division of “sub-fossil birds” embraces birds that have existed during the human period, but have now disappeared. In what are called the “kitchen-middens”[45] of Denmark, and in the lake dwellings of Switzerland, in recent fens and peat mosses, and in various islands, as Madagascar, New Zealand, and the Canaries, the remains of the great auk, the capercallzie, the dodo, the crested parrot, and in New Zealand the huge dinornis have been found. Within the historic period various other birds have been exterminated, and are now represented only by ancient drawings, stuffed skins, or relics of their nests and eggs.[46] FOOTNOTES: [42] These differential characteristics apply only to existing species of birds and reptiles. [43] See Nature Reader, No. 3, Lessons 28-30. [44] This adaptation does not indicate any descent of the bird from the reptile. [45] The word means “kitchen refuse heap” and is applied to shell-heaps mingled with bones, found along shores and recognized as traces of ancient human habitations. [46] See Nature Reader, No. 3, Lesson 39. LESSON XX. THE EARLY MAMMALS. “’Tis there the otter dives, the beaver feeds, Where pensive osiers dip their willowy weeds, And there the wild-cat purrs amid her brood, And trains them in the sylvan solitude, To watch the squirrels leap, or mark the mink Paddling the water by the quiet brink.” --BRAINARD.