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and oxygen, together with water, and it is not found except in living bodies. Some of its simplest constituents can be made by chemists artificially, but the artificial manufacture of protoplasm is not yet possible, and there appears no probability of its being obtained except from living substances.

Living things show a successive change and development, leading to the decay of the individual; but

SWIMMING PROTOPLASM

during this development the individual is renewed by some process of reproduction or division.

Life gives the power to receive from food a supply of energy and the power to do work.

Life requires a special environment. Certain conditions of temperature and moisture must be provided if it is to survive.

Living things grow by food different from themselves. Living things adapt themselves to their surroundings in a way that tends to self-preservation.

By these characteristics, which are not shared by inorganic matter, we can distinguish what is living from what is not living.

All forms of life must have evolved from the simplest possible organism-a drop or grain of protoplasm.

Now, as has been said before, many millions of years ago the temperature of the earth was such that no life, such as exists to-day, not even a grain of protoplasm, could have existed. How then did life come to the

We can only form theories.

earth? We do not know. Some authorities have held that life is as old as

matter. If this be so, protoplasm, which, as it now exists, is a highly complex substance, probably passed through a series of simpler stages; and in these stages it must certainly have been able to withstand conditions of temperature which, as far as our knowledge goes, it cannot now endure. Otherwise the grain of living protoplasm must have reached our earth when it had cooled and solidified sufficiently to be a fit abode for life. It has been suggested that it may have been carried in the crevice of a meteorite-that is, what we call a shooting star. Protoplasm, however, can withstand extremes neither of heat nor cold, and a meteorite would be exposed first to the intense cold of outer space and then to intense heat, due to friction, as it passed through our atmosphere. Moreover, we have no reason to assume that the conditions of the early earth were not as suitable for the first development of life as those of any other of the heavenly bodies.

It may be that living matter evolved in favourable conditions from non-living matter very long ago, but after the earth had passed from a stage in which life could not exist. At the present day, as far as we know, there is no generation of living from lifeless matter, but it is possible that there were as many stages between the non-living material of life and the living protoplasm as there have been between the protoplasmic grain and the myriad forms of life that evolved from it. If it be true that life was evolved from the lifeless matter of the earth's crust, then it is in a literal sense true that man is made of the dust of the ground, since he is the culminating point of the many-branched tree of life that sprang from it.

What were probably the conditions on the early earth that led up to the wonderful day when life was born?

The surface was warm and moist, and owing to the density of the atmosphere the temperature would probably vary very little day and night all the year round. While the earth was hot enough to be incandescent various nitrogenous and other compounds would be

formed which would readily break up and form new combinations. When the water was condensed upon the surface of the earth it would contain in solution materials from the earth's crust and also gaseous compounds of carbon, nitrogen, and phosphorus from the atmosphere. The water with its dissolved salts and gases would saturate the mud along its shores, which in temperature and moisture would provide a suitable environment for the protoplasmic jelly. Sunlight would provide the energy necessary for the breaking up and CLOSED IN CELLS recombination of the mineral salts and carbon compounds, and thus might originate very simple living substance, developing by slow stages into protoplasm.

[graphic]

PROTOPLASM EN

The simplest organisms consist of single cells-i.e. of a minute amount of protoplasm surrounded usually by a skin or cell-wall. Some organisms have remained in this stage; all arose from it, and one of the greatest steps in evolution was that from single-celled to manycelled organisms.

Some single cells combine with others to form a manycelled body. Here there is no development of cell from cell. Cells subdivide owing to the influence of their

nucleus, a more compact mass of protoplasm in the centre, an essential constituent of which appears to be phosphorus. Without this cells may live, but they do not subdivide. Sometimes the nucleus divides again and again within the cell, so that an organism with many nuclei is

[graphic]

formed. Sometimes the nuclei break the wall and begin a fresh life as single-celled organisms. Manycelled organisms, or bodies, were probably formed as the result of a failure to achieve this separation. When the stage of growth arrived at which division into two or more separ

ate organisms usually occurs the

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[graphic][merged small]

nuclei, unable to break out and start an independent existence of their own, remained in association with the mother-cell and originated a growing cellular structure or body. It was a great stage in evolution which associated life with a body, for it was the beginning of all the different forms of life.

The simplest of all living creatures are called by Haeckel 'protists.' We do not know how close to or how far from the first organisms they are, but they are like them in this, that they are neither distinctively

plants nor animals. Gradually the life of the world divided into these two main streams. The protists, as it were, have never been able to make up their minds to which stream they belong.

When we considered what we meant by life we found that the distinction between organic and inorganic was not as clear as we should have expected, and while we were conscious of a profound difference between the two we discovered resemblances which were almost as striking. In the same way we shall find important resemblances and marked differences between plants and animals. If we investigated sufficiently far we should find the one branch of life merging into the other by stages so subtle that at last we could not say, "This is plant," or "This is animal."

Plants and animals alike breathe, feed, digest, grow, have families, and die. They are alike in being built up of cells and modifications of cells, all originating in one fertilized egg-cell.

They are totally different in the way they obtain their food. All the materials necessary for the food of both exist in the air, water, and earth. For everything except water animals are dependent directly or indirectly upon plants. The carbon and nitrogen they require they obtain, not from the air or the soil, but from complex organic compounds built up in plants or in other animals who have obtained their materials from plants. Plants get their carbon from the carbon dioxide of the air; in the presence of sunlight this is built up within their tissues into various carbon compounds. Their nitrogen they take in by their roots in the form of nitrates in solution. Sun-energy is used by the plant in building up complex food-stuffs which contain potential chemical energy. The animal eats

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