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

THE EVIDENCE

Belgian cave, obviously entombed by some accident.

Fossil ants, we note, are often found in amber. Amber is the fossilized gum which once exuded as resin from long decayed pinetrees. Only an insignificant fraction of the ant population of the world gets trapped in resin ; and only an insignificant fraction of the resin is hardened and preserved as amber ; and yet almost all our knowledge of the ants- of the past is derived from specimens in amber.

The amount of detail preserved to us is also very much a matter of luck. As a rule, nothing survives but the skeleton, and even that may be distorted and sometimes partly rotted. Now and again, however, happy accidents have caused traces of the softer parts to survive to our day. For example, one or two specimens of the strange, duckbilled dinosaur, trachodon, in the Upper Cretaceous (IV C3), died and fell on a patch of soft mud. They decayed and the place where they had been became covered with sand. But the mud beneath held the impression of their skins with surprising fidelityso that now we have the form of their skins preserved, with a mould in hardened mud and a cast in hardened sand. Some of the dolphin-like ichthyosaurs left records of their flesh and fins; some even of their feces, marked with a spiral twist by the folds of their intestine, and containing undigested remains of the fossil squid-like creatures called belemnites.

The very first known land beasts left footprints in the mud across which they lumbered; the bird-reptile Archzopteryx stamped its feathers with astonishing detail in the fine-grained lithographic limestone of Solenhofen.

But there are even older happy chances than these. Some Devonian plants (III D) are so minutely petrified that we can study the precise shape of the microscopic hairs protruding from their leaves. And recently Walcott has discovered a wonderful array of invertebrates imprinted on Middle Cambrian shales (III A 2), nearly as clear to view as when they were swimming about, ages before the first fish, almost five hundred million years ago. The soft mud, now pressed hard, reveals to us the outlines of jelly-fish, annelid worms, with their appendages and bristles, small crustacea, with all their soft leaflike appendages, even the outlines of their stomachs, and arrow-worms, just like those that swim in our modern seas, with their transparent fins. Walcott has also found swarms of bacteria from. still

OF THE ROCKS

earlier rocks. It is doubtful whether this is to be considered the most extraordinary case of fossilization ; it is certainly rivalled by some Paleozoic (III) fishes (to be seen in New York) which have been so delicately petrified that thin slices of their muscles, ground down to transparency, and looked at under a high power of the microscope, show the cross-sections of the muscle-fibres as clearly as a fresh-made preparation from a modern dog-fish. How the microscopic structure of living tissue came to be thus translated into stone we do not fully understand ; but at least the rarity of such lucky finds brings home to us the multiplicity of what is lost for ever.

The various difficulties thus put in the way of paleontologists are of two main kinds. There are those which lead to gaps and imperfections in the fossil record, whether through the fewness of animals and plants which became fossilized, the washing away of large sections of the crust when brought above water, for wind, rain, and frost to destroy, or the destruction of fossils by the heat and pressure of metamorphosis, And there are those which make it difficult to arrange what fossils we have in the right order, whether the difficulty springs from the turning of layers topsy-turvy and from faulting, or from the fact that two contemporaneous layers might show quite different fossils, either because they were laid down in quite different situations, or because, though comparable in the environment they provide, they were situated far apart on the earth’s surface. F

The imperfection of the record is unfortunate, but nothing more. It makes the labours of fossil-hunters greater, and should warn us against clamouring for the immediate discovery of this or that ‘* missing link. But patience and the exploration of more and more of the earth’s surface are bringing their own reward. Every year the record becomes less scrappy, and in many groups of animals, what fifty years ago seemed impossible to hope for has been achieved,

and an unbroken series discovered, leading

through ages of time from simple to highly developed types. The difficulty 1s not one f principle. ' " at i other difficulty seems at first sight more serious, and the opponents of Evolution, anxious to find any stick to beat a dog with, have tried to make out that it is a defect of principle. The Evolutionist, they say, pres tends that he dates his fossils by the order o the rock-layers in which they are poe but in reality he very often dates his rock: 201