away by rivers. These constituents are washed down from the primary rocks of the mountains, or carried by the wind over wide areas in volcanic dust, or provided by the decay under denudation of the volcanic rocks themselves.

Great clouds of gas and steam are mingled with the molten rock in a volcanic eruption. The hot waters dissolve some of the metals with which they come in contact, and when they cool the metals, which had before been too scattered to be useful, are deposited as mineral veins, from which they can be mined.

The last stage in the breaking up of both primary. and secondary rock is the formation of soil, which is the topmost layer of the earth's crust. It is the rock in its final condition of decay, the agents of which are chiefly water and the gases of the atmosphere. To-day burrowing animals and earthworms and the roots of plants are always helping the work of pulverization. Soil, however, had to be formed before the higher plants could exist at all, since they must have a layer of loose material into which they can thrust their roots and fix themselves to the ground. There must be soluble plant food too, for plants can take only in solution that part of their food which comes from the soil. They have the power of obtaining carbon from the carbon dioxide in the air and using it to build up the carbon compounds in their tissues. Nitrogen, another essential element in plant food, exists as a free gas in the atmosphere, but cannot be assimilated by plants in that form. They must obtain it from mineral nitrates in the soil. The formation of these is aided by minute and primitive organisms known as bacteria. The plant uses the nitrogen from the nitrates in building up products which can be assimilated by animals, for animals

are dependent upon plants for nitrogenous food, as well as for carbon compounds.

As soil, which is rock broken down to the uttermost, is essential to plants, and as without plants there could be no animal life, it is the soil which is the first source of our food supply, and on which all life depends. It is the end finally reached by the once molten rock of the earth's crust.

No life, as we know it, could exist when the primary rocks were solidifying from the molten state. Evidence of the different types of life that came into existence later is found in the secondary rocks that were being formed when they lived and in which they were buried. Type succeeds type, the lower giving way to the higher, or surviving alongside it because of some necessary function that it fulfils. The higher forms of life were possible only because the more primitive preceded them and prepared their way.

I

CHAPTER IV

THE COMING OF LIFE

T has been said that secondary rocks contain the remains of the animals and plants which were

living at the time that they themselves were being laid down. These remains are called fossils. The oldest fossils that we possess, however, are those of plants and animals which must have had a long line of predecessors. They are the fossils of plants and animals with hard parts-animals with shells or skeletons, and plants with woody stems. Such parts are not found in primitive organisms. They are adaptations of structure to meet a change in environment. Shells, for example, are defensive armour evolved-we cannot tell by what stages-to resist the attacks of an animal that has become carnivorous. Skeletons give the power of swift movement, which has become necessary so that their possessors may escape from enemies or pursue a prey. Such specialized organisms must, it is clear, have been preceded by simpler ones, and the soft tissues of the earliest organisms would not be preserved as fossils, even if the mud on which they were formed had not in all probability been destroyed by the unresting forces of denudation and rebuilt into new rock. We cannot then expect to find in the earth's crust records of the earliest organisms, but there are in existence at the present day organisms so simple that we may believe that the earliest forms of life resembled them. As to the manner in which

life itself originated we can at present only form

theories.

So far we have spoken of life without feeling the necessity to define what we mean by the word, because every human being with the power of thought recognizes life as something existing in himself. He is aware of it in that very faculty of thought—that is, in his consciousness. When, however, we come to define 'life,' to distinguish between what is organic or living, and what is inorganic or non-living, we shall find it a difficult matter. We shall be reduced, in fact, to stating some of the characteristics of life without saying what life is. Does life reside in consciousness-that is, in the faculty of thought? We know that this faculty exists in ourselves; we assume that it exists in other people whom we see acting in given conditions in similar ways. But as we descend the scale of life we see less and less of what we can interpret as thought, and in the lowest forms of life it does not appear to exist at all. If all life evolved from some simple organism, it is more probable that consciousness developed gradually, as the forms of life became more complex, than that it is always associated with life.

Living things-that is, plants and animals-take in food. They keep what they need to build up their tissues and reject what they cannot use, and the result is growth. But mineral crystals also grow; they take from solutions what they can use as food and leave what is useless to them, so that the absorption of food and the function of growth do not mark off what is living from what is non-living.

Organisms, however, it may be said, absorb food internally and have an inner growth, while crystals are built up on the outside, and this is a general distinction

between living and non-living matter. Yet there are some inorganic bodies which also have inner growth and which under mechanical stresses produce forms which have a strong resemblance to the leaves and stems of vegetable life.

Changes of structure and composition, it has been said, take place in a living individual without its losing its identity. The same is true of the growth of crystals, successive layers of which may be different both in composition and structure, and yet the crystal retains its form and is recognizable as the same.

Living bodies have the power at a certain stage of growth of dividing into, or producing, smaller bodies which display the same characteristics. Crystals also have a limit of size, and when that is reached, if fresh material is deposited on the surface, a younger generation of crystals is formed, growing on the same lines as those that preceded them until they reach their greatest size, when they too produce a fresh generation.

So far, then, we have not found any satisfactory line of cleavage between what is living and what is non-living. Inorganic matter has no consciousness. But consciousness is not found in all forms of life.

Crystals as well as living bodies take in food and reject what is useless in it, grow, and have the power of reproduction, and undergo changes in structure and composition without losing their identity.

It is difficult, then, to define the difference between the living and the non-living, so difficult that we are forced to give up the attempt to do so and must try to describe life by its characteristics.

The substance of living things is a jelly-like material called protoplasm. This consists of very complex compounds of carbon, hydrogen, nitrogen, sulphur,