common with the warm-blooded, feathered inhabitants of the air; and yet our first scientists, for instance, Prof. Huxley, unite reptiles Fig. 2.—Section of the Earth's Crust. and birds as one class of animals. However closely we may study the anatomy of a chicken and of a crocodile, and search for points which are common to both, we shall be only moderately inclined to follow the example of these naturalists and consider birds and reptiles as near relatives. But we shall become convinced of their close connection if we study also their geological history.

In the first instance, we then see that the reptiles of the present time are only small remnants of a once numerous and powerful tribe. In certain former geological periods we find reptiles to be the kings of the earth; we discover reptiles of enormous sizes; reptiles of all kinds of shape and anatomical construction; and among them there are some which resemble birds much more than does a crocodile or a lizard of the present day. In the second instance, if we look at the birds of former periods we find that, in the same way as we go backward in the history of our earth, these former inhabitants of the air differ more and more from the specialized pattern after which all our present birds are built, that they become more and more reptile-like, and that there can hardly be any doubt that the birds are indeed branched off from the great reptilian trunk of the animal kingdom.

There are three fossil birds of pretertiary age known almost completely; two of them were found in the Cretaceous formation of North America, and one in the Jurassic formation of Germany.