There are only a few obvious differences between birds and reptiles. To answer the demands of flight, birds have achieved a constant body temperature and a metabolic rate higher than that of reptiles. For lightness some of the bones have become hollow. A breastbone has developed for attachment of the flying muscles, and bones of the forelimb have been lost or bound together to support the wing along its leading edge. The teeth and the long reptilian tail are gone. Looked at with these differences in mind, Archaeopteryx is a wonderful example of an intermediate creature. Its jaws were toothed, its tail was long, its breastbone was weak and all its bones were solid. Its power of flight was surely feeble; but it had feathers, and it stands as a beautifully explicit link and sign of kinship between the nightingale and the crocodile. Archbishop Ussher himself, gazing upon Archaeopteryx, would embrace Darwinism, and would know at once where the birds came from.
The record of the derivation of mammals from reptiles is both far longer and far more detailed than the history of birds and also, unfortunately, it is far less clear. It begins with the pelycosaurs of the late Carboniferous-- a group not far removed from the old stem reptiles. From these there radiated a great array of types known as mammal like reptiles, and during the late Palaeozoic and early Mesozoic these creatures were the dominant vertebrates on the land.
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