To the lay mind the distinctions between the Amphibia and the reptiles are not very obvious, and indeed in the older classifications the former group was not separated from the latter. The differences between a reptile and a bird, on the other hand, are very striking. It might therefore be regarded as a matter for surprise that zoologists now make the greater distinction between the Amphibia and the reptiles, grouping the former in one great class with the fishes, the latter in a second great section with the birds. But in fact there are many fundamental points of agreement between reptiles and birds, and it is impossible to doubt that the latter have sprung from a reptilian stock. Indeed, a most interesting connecting link is known, in the fossil Archiopteryx shown in Fig. 98, of which only two specimens have been found, and which is the only creature of its type of which we have any record. In all its skeletal features, the Archiopteryx is reptilian, and it would undoubtedly have been classed as a new type of reptile but for the obvious and unmistakable traces of feathers. From what particular class of reptiles the birds have sprung is not known. The birds have assumed the position of almost unquestioned masters of the air, but like other great groups they show possibilities of evolution in other directions wherever opportunity offers, and types like the kiwi and the penguin shown in Figs. 100 and 101 have forsaken their native element--the one for the land, the other for the water. The birds agree with the mammals in the development of a four-chambered heart, in their warm blood, in their external covering for the skin, and in the development of arrangements and instincts for the parental care of the young. Their line of Evolution has thus been to some extent parallel to that of the mammals. On the other hand, they differ obviously in the structure and function of their fore limbs, in the absence of a diaphragm, and in their special methods for the care of the young, and there can be no doubt that the two groups have had quite different origins in the reptile class. [Illustration: FIG. 98.--Archiopteryx, a fossil lizard-like bird. Photo: Underwood.] [Illustration: FIG. 99.--Eagle, with prey.] [Illustration: FIG. 100.--The Kiwi, Apteryx. A practically wingless running bird. Photo: W. P. Dando.] [Illustration: FIG. 101.--The King Penguin. A highly specialised diving bird. Photo: W. P. Dando.] CHAPTER VI THE MAMMALS AND MAN