IN a paper read to the Zoological Society in 1880, Huxley remarked that the astronomer who had determined three places of a new planet could calculate its place at any epoch, however remote and if the law of evolution was to be depended upon, the zoologist who knew a certain length of the course of that evolution in any given case, might with equal justice reason backwards to the earlier but unknown stages. He accordingly surveyed the backboned or vertebrate animals, both living and extinct, so far as they had then been discovered, and he defined the successive stages through which they must have passed before they ended in the higher warm-blooded quadrupeds and birds which dominate the world of life to-day. He pointed out how, in the existing world, most of the earlier stages are represented only by animals with very special adaptations to their several modes of life, which give little idea of the variety displayed by animals of the same stage in former geological periods. He concluded that a multitude of extinct groups of each stage had still to be revealed, most of those groups being short-lived adaptations to the several spheres of life which were open to them at the time, and only a small proportion on the direct line of ancestry of the existing vertebrate animals. The actual links in the chain of life from the lowest to the highest rank must, therefore, have been comparatively few.