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

BOOK 3

elongation needed, if the animals were to continue rooting in the ground with their lower jaw; became mechanically impossible ; and at the same time the head, with its great tusks, was too heavy to be borne on any but a stout and short neck. Accordingly, evolution changed its direction and pushed forwards towards a short, tuskless lower jaw, while the development of a trunk kept the animal in touch with the ground. And other series almost equally perfect can

Fig. 128. A bird with teeth like a reptile.

Its remains are found in rocks of the Cretaceous Age (IV C). over five feet in length.

be found among various groups of carnivores. In reptiles, the fossil crocodiles gradually shift the internal openings of their airpassages farther and farther back along the palate, making it easier for the animal to breathe while holding prey under water in wide-open jaws. Moreover, series of smaller scope but greater continuity, like that of Micraster, are now available for certain

starfishes, lamp-shells, ammonites, pondsnails and other invertebrate forms. Sometimes the fossil record is not so

212

‘THE SGIENCE OF LIFE

The extinct diving-bird, Hesperornis, as it probably appeared in life.

It was

GHAPTER 2

complete, but yet discoveries, though isolated, may be of startling interest as supplying the “missing links,’ as our grandfathers called them, between hitherto isolated groups. Typical of such linking types is the primeval bird, Archeopteryx. Two almost perfect skeletons of this creature are known, both from the Jurassic Period (IV B) in the middle of the Age of Reptiles. The rock in which their form is preserved, at Solenhofen, in Bavaria, is so fine-grained that it is used for lithographic stone and has retained the smallest details

down to those of the delicate feathers. Birds, as was suspected even

before the discovery of this missing link, are descended from reptiles ; they are reptiles which have been specialized for an aerial life.

In Archeopteryx we see the specialization in progress, incomplete. In a modern flying bird, for example, the reptilian fore-legs are turned sideways to serve as wings, and this has necessitated profound changes in their structure. Several originally distinct bones are welded together for rigidity, and the claws are lost (except in the young South American Hoatzin, which uses its wings to clamber about in the bushes). But in Archeopteryx, although the forelimb is very certainly a wing, the welding of bones had not yet been brought about and there were still three well-developed and movable clawed fingers protruding from the wing, used presumably in climbing. Again, in a modern bird the tailskeleton is short, a stumpy little support for the fan of tail-feathers, but Archeopteryx had a long lizard tail with twenty vertebre or more, and a row of large feathers on each side along its whole length. And instead of the horny, toothless beak of a modern bird, this winged, feathered lizard had ordinary reptilian jaws with a fine array of socketed teeth. Had it not been for the happy accident by which the feathers were preserved, it is doubtful whether the skeleton alone would have warranted us in definitely calling the creature a bird. As it is, Archeeopteryx is in its general construction a perfect link between the two great groups of birds and reptiles, though more than half-way to modern birds.

Its wings were too small to sustain its