what the plant has stored up. He breathes—that is, takes oxygen into his lungs. Within his frame the oxygen combines again with carbon and hydrogen and is given out as carbon dioxide and water. The potential energy stored in the plant-food is set free within his body in the form of animal heat and animal motioni.e. locomotion and other forms of activity. The animal has spent the sun-energy stored up by the plant. The relations existing between plants and animals, plants being storers of energy of which animals are the consumers, lead to one of the great distinctions between plants and animals. Animals in general are more active than plants; and again, because of their dependence upon plants for food, animals need the power of free locomotion.

It is unnecessary after this to ask which came first, plant or animal. No animal could exist until plants had first prepared stores of food for it. The earliest living organisms were indistinguishable as plant or animal until some of them began to manufacture chlorophyll, the green colouring matter of plants, without which they cannot build up the complex carbon compounds that animals require. The main outward sign of a plant is that it is green, and this it owes to chlorophyll, the production of which made possible the life of plants, and through them of the lower animals and man.

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CHAPTER V

EVOLUTION

INCE this globe, this earth of ours, moulded into every variety of surface, crowded with innumerable forms of life, arose in dim, unimaginably remote ages in a nebula, and passed through a stage when it was fluid and so hot as to give out light, it is clear that it has undergone constant and continuous change. It is still undergoing it. This orderly change and development, not only in the earth but in the universe, in the smallest things as well as in the greatest, in the present as well as in the past, in the earth's crust, in plants and animals, in the minds of men, and consequently in their laws and customs, and in their struggles to maintain themselves upon the earth and to become masters of their environment, we call evolution.

We are constantly seeing evolution at work both in history and in everyday life. We can trace, for example, the development of the modern English Parliament from the Witanagemot of the Saxon kings, or that of the modern trade unions from the medieval gilds of craftsmen. We can see before us in the course of a lifetime or from models in museums the evolution of inventions which sprang from the minds of men. Every change of construction is intended to bring about an improvement in some part where the original invention was weak, and the new forms either have to establish themselves as better than the old or they are soon relegated to the scrap-heap. Thus we can follow the

steps by which this velocipede or ' bone-shaker' became the modern bicycle, the first armoured vessel the latest super-Dreadnought, and Stephenson's "Rocket" the present-day locomotive engine.

Again, we know that if a gardener wishes to produce a new colour in a certain flower he takes his seeds from the plants which approach most nearly to it, and raises his seedlings from these. When the new generation of plants is in flower he again selects those individuals for his experiments which show perhaps a deeper shade of the desired colours, and by repeating this process many times he can usually attain his end. In this way a new variety is produced as a result of the deliberate selection by the grower of the plants from which to take seed.

Because such facts as these are familiar to us, and because the doctrine of evolution is part of the thought of educated men at the present day, it is surprising to find that it has been generally held for not much more than half a century.

It is true that we find in the writings of certain Greek philosophers as early as the seventh century B.C. allusions which show that they possessed an inkling of the principles on which the modern theory of the universe is based. Their teaching, however, was vague, because it could not be founded on that detailed knowledge of nature which it took more than another two thousand years to gain, and which required the invention of scientific instruments. Thus when, at a later date, Heraclitus says, "All is flux "--that is, "All is change" we assume that he had grasped that idea of the ceaseless development of the universe which we call evolution, but we cannot be sure of it.

Again, another Greek philosopher, living more than five hundred years before Christ, had sufficient insight

to declare that the fossils of plants and animals found in rocks were the actual remains of formerly living creatures, and that the rocks in which they were found must have stood at some time under water. Yet throughout the Middle Ages and even down to the eighteenth century the most absurd ideas prevailed as to the nature of fossils, as, for example, that they were produced by the influence of the stars, and thus the illuminating knowledge was missed that in the fossil remains of plants and animals preserved in the earth's crust, the comparative age of which can be estimated by the depth of the rock layer in which they are found, we have a wonderful record of the successive forms of life that have existed on the earth since it became a fit abode for life at all. The great painter Leonardo da Vinci in the fifteenth century stated the truth about fossils, but in this as in other matters he was too far in advance of the age in which he lived to gain many followers. Moreover, the Inquisition made it dangerous to announce new theories in Roman Catholic countries. In 1600 the Dominican friar Giordano Bruno was burnt at the stake in Rome for expressing evolutionary ideas about the universe, and in 1633 his countryman, the great astronomer Galileo, was three times summoned for examination before the Inquisition, and forced under threat of torture to recant similar convictions.

As late as the eighteenth and early nineteenth centuries people in general held much less enlightened views concerning the making of the earth and the development of the life on it than those of some thinkers among the Greeks more than two thousand years earlier. Early in the nineteenth century the great French naturalist Cuvier published a famous work on

the fossil bones of backboned animals. In this he states that the extinct species of animals whose remains are found in the different layers of the earth's crust differ the more strikingly from the living species of animals akin to them the deeper those layers lie-that is, the more remote the ages in which those animals lived. This, of course, is true and what we should expect, since the fossil animals are the ancestors of the present living animals, or the representatives of kindred lines which became extinct. Unfortunately Cuvier wrongly believed that the same species were never found in succeeding groups of rock layers, and he therefore held that a series of unexplained catastrophes had periodically destroyed the whole world of animals and plants and that at the beginning of the next period a new and improved population of living things had appeared. With these ideas the majority of people were satisfied. Yet evolutionary ideas were in the air, and in the early years of the nineteenth century we find Englishmen such as William Charles Wells and Patrick Matthew, Germans such as Goethe, Treviranus; and Oken, and Frenchmen such as Geoffroy Saint-Hilaire and Jean Lamarck all publishing theories of the origin of life and species corresponding with modern views on the subject. Of these Lamarck developed his theory most fully, in works published in 1801 and 1809, and he was so much in advance of his age that they were understood by few and disregarded by the world for fifty years.

In 1830 the way was paved for the man who was to win general acceptance for the evolutionary theory of the universe by the publication of Lyell's Principles of Geology. In this he showed that Cuvier's theory of unexplained catastrophes changing the surface of the earth and wiping off the life on it was unnecessary and