The science of life : fully illustrated in tone and line and including many diagrams

-a tortoise, Eunotosaurus, with well-

THE EVIDENCE

whole weight, and so the hind part of the body had to be supported (as in almost all aeroplanes to-day) by a large tail-plane. Archzopteryx must have lacked the power of rapid, controlled flight ; that came only with stronger arm-skeleton and larger wings which allowed the tail to be reduced and so permitted the bird to turn, check, and drop suddenly, instead of planing along and coming to grief below a certain speed as an ordinary aeroplane will do.

But there are other linking fossil forms in the bird stock. In the Cretaceous Period (IV C) birds have been discovered which were essentially similar to living birds in wings and tail, but still had teeth on the jaws (Fig. 128). Before the Middle Eocene (V A 2) these toothed birds had disappeared, and from thenceforward all bird fossils are of modern type. A somewhat similar example comes from reptiles. All modern tortoises and turtles are toothless and beaked like modern birds ; but far back in the Permian Period (III F) there lived

formed teeth like the majority of reptiles.

The mammals, too, are now linked by fossils with their reptilian ancestors. We knew already that mammals must have sprung from reptiles. Apart from all other lines of evidence, the discovery of those “living fossils,’ the Platypus and the Echidna, clinched the matter. Had we nothing but the skeletons of these animals, it would be very doubtful whether we should call them reptiles in the last stage of becoming mammals, or mammals which had just ceased being reptiles. But the fact that they nourish their young by a milky secretion, possess a coat of hair, and have a more or less constant temperature, stamps them as true mammals, even though their egg-laying habits and many other peculiarities show by how little they have crossed the boundary.

This is convincing proof ; but all the same it is only indirect. However, in the Permian (III F) and Triassic (IV A), as the Age of Reptiles was dawning, a group of creatures existed whose name of Theromorphs implies a likeness to mammals. ‘Their teeth

OF THE ROCKS

were already beginning to differentiate into incisors, dog-teeth, premolars and molars, and the whole form of the skull was approaching the mammal. They did not crouch on their belly, but ran with body lifted off the ground. But they still retained the little

Fig. 129. Steps in the evolution of ear-bones from jawbones in the mammal-like Theromorph reptiles.

Top, skull of Scymnognathus from .the Upper Permian (HI F 3).

Bottom, Cynognathus from the Lower Triassic (IV A1). (D): the

bone which forms the jaw in mammals. :

in most repliles form the joint between upper and lower jaws, a

mammals become the little Anvil- and Hammer-bones in the ear. (T) : the bone on which, in mammals, the ear-drum 1s stretched.

(A) and (H): bones which nd in

hole in the top of the head, the pineal foramen, below which, in many modern reptiles, the third or pineal eye 1s still to be found; this has been lost in all known mammals, with the transformation of the third eye into the pineal gland.

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