THE ANCESTRY OF BIRDS. 617 



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 sepa- 

 rate, 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 archaeopteryx, 

 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 swell- 

 ings inside the jaw, known as dental papillae, 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 devel- 

 ment as though it meant to start a set of conical fangs in its jaw, 

 like those of the archaeopteryx, 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 rep- 

 tilian 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 vertebrae, 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 Provencal 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. 



