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

BOOK 3

8 4

The Continuity of Evolution as Shown by Sea-Urchins

One great merit of the horse’s evolutionary record is that the animal is familiar and that we can readily understand the biological meaning of the main trends in its long, ancestral development. Its only defect as a demonstration is that the record is nowhere continuous in one single locality.

If we could find a considerable thickness of rock, all deposited under approximately the same conditions, we should expect to find an absolutely unbroken sequence, a still more unbreakable evolutionary chain, in the fossils which it contains. Such large thicknesses of one kind of deposit are naturally, though unfortunately, rare ; for usually, as deposits pile up they are brought nearer to the surface of the sea by the mainland rising, or by their own gradual accumulation, or they are submerged deeper by the land’s sinking. In either case the character of the sediment, and therefore of the animals which can live at the bottom, will change, since, for instance, sand particles will sink before fine mud, and future sandstones therefore be laid down nearer inshore and in less quiet waters than future clays. .

But the chalk of the Mesozoic Epoch (IV) happily serves our purpose. Up to a thousand feet of it were continuously laid down in large but shallow seas, originally as a limy mud, largely formed of the tiny skeletons of single-celled animals rained softly down from the waters above. ‘This went on during much of the Upper Cretaceous Period (IV G 3), for at least ten million years ; and through much of this time conditions of life on the bottom of the chalk-depositing sea continued so similar that many of the same kinds of animals are found in every layer.

Among the most abundant and the best studied of the chalk fossils are the seaurchins known as Micraster. These are found, and found abundantly, throughout most of the lower halfof the chalk. In Southern England, for instance, they persist through 450 to 500 feet of chalk, the total thickness of the deposit varying from nearly 1,300 to nearly 1,500 feet. ‘Translated into time, this means 35 per cent. to 40 per cent. of the total chalk period, certainly over four million years.

Throughout this long period the fossil Micrasters are so abundant that hundreds of thousands can be collected and a gradual evolution can be traced as we pass upwards. The changes are apparently trivial. ‘There is a slow alteration of shape from rather

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THE SCIENCE OF LIFE

CHAPTER 2

flattened to rather arched, and from rather elongated to about as broad as long. The mouth creeps steadily forward, its distance from the front border of the lower surface decreasing from about a third of the bodylength in the early types to a sixth in the latest, on a total length of fifty to seventy millimetres. A low ridge, totally absent at first, appears and grows slowly higher along the hinder part of the upper surface. The grooves from which the tube-feet emerge grow longer, and their surface, smooth at first, becomes sculptured. The mouth gets more and more overhung by a protruding lip of the hard skeleton; the little knobs on the skeleton become in some regions gradually more prominent. There are other changes, but these are the most obvious.

Opinions differ as to the number of separate species into which Micraster should be divided during this period. Since the series is continuous this question can only receive an arbitrary answer ; but the most conservative estimate is half a dozen.

The changes are infinitesimal, both in extent and biological meaning, compared with those in the horses. The explanation doubtless lies in the fact that at the beginning of the chalk period the sea-urchins were already an old and well-differentiated stock. Even the earliest Micrasters were highly specialized for life in sand or mud, and there was neither need nor room for any radical improvements. The mammals, on the other hand, were one and all primitive and small in early Cenozoic times ; there was obvious necessity and opportunity for their improvement, and the horses shared in the great evolutionary movement which completely remodelled the whole mammalian stock between Eocene (V A) and Pliocene (V D).

Steadfast as it is, Micraster answers our present purpose, for though the changes involved are small, they are absolutely continuous, the urchins found at one level grading quite imperceptibly into those of the rest; a single specimen, indeed, may show characters of one “‘ species” in some of its tube-feet grooves, characters of another in the rest. There is no question but that the Cretaceous sea persisted without any notable change in its conditions throughout the whole time, and that our urchins lived, died, and reproduced upon its bed in a continuous succession, so that the fossils from the lower layers were actually the parents of those embedded higher up. Slowly the race modified itself; by almost imperceptible degrees Micraster changed itsshape. The evolutionary movement which Micraster demonstrates