glands with whose contents the birds preen and dress their shining plumage, to secure them against the evil effects of damp or rain. But, while the young chick is in the egg, all its tail-bones still remain separate, as in the ancestral, lizard-like bird and the still earlier ancestral lizard; it is only as the development of the embryo progresses that they become firmly united, as in modern forms. In other words, every young bird begins forming its tail as if it meant to be an archæopteryx, and only afterward so far changes its mind as to become a crow or a sparrow. Similarly, no adult existing bird has true teeth; but the young of certain parrots show in the egg a set of peculiar little swellings inside the jaw, known as dental papillæ, and commonly found as the first stage of teeth in other animals. Moreover, these swellings are actually covered by a thin coat of dentine, the material of which true teeth are made. So here again the young parrot begins its as though it meant to start a set of conical fangs in its jaw, like those of the archæopteryx, but afterward changes its mind and contents itself with a bill instead. Such symptoms as these point back surely though remotely to a far-distant reptilian ancestry.
It is worth while noting, too, that the links which bind the birds to the reptiles bind them also in part to the lower mammals. For the lowest existing mammal is that curious Australian creature known to the rough-and-ready classification of the colonists as the water-mole, and rejoicing in the various scientific aliases of the ornithorhynchus and the duck-billed platypus. Unsophisticated English people know the animal best, however, as "the beast with a bill." Now, there are many close resemblances between this strange Australian burrower, on the one hand, and such antiquated forms of birds as the New Zealand kiwi, on the other. In many particulars, too, the water-mole recalls the structure of reptiles, and especially of the ichthyosaurus. In short, it is at once the most bird-like and the most reptile-like of mammals. Hence we may fairly conclude that birds and mammals are both descended by divergent lines from a single common reptilian ancestry. For, on the one hand, the kiwi, an early type of nocturnal bird, preserved for us in isolated New Zealand, has some marked reptilian and mammalian affinities, not only in the external character of its hair-like feathers, but also in the more important structural points of its diaphragm, its movable vertebræ, and its keelless breast-bone, which are questions rather for the professed anatomist than for mere idle loungers basking lazily in the sun on a Provençal hill-side. And, on the other hand, the ornithorhynchus, an early type of burrowing aquatic mammal, preserved for us in isolated Australia, has marked reptilian affinities in its bony structure, and in the teeth implanted on its tongue; while it has also marked resemblances to the ducks and other swimming birds in the external features of its horny bill and webbed feet, besides being still more closely related to them in many of its less obvious anatomical peculiarities.