they had done so it is probable that the beds of rocks in which they were buried would have been destroyed in the course of the ages that have lapsed since they were laid down. Our earliest plant fossils, however, belong to the class of water plants, or algae, one of the two primitive groups-the other being the land group of fungi (mushrooms, toadstools, etc.)—which have no distinctive structure for stem and leaf, the whole body of the plant being a mass of simple cells, and it is probable that these formed the only vegetable life on the earth for more than half the period between the appearance of the first living cells and the present time

a vast period comprising probably many millions of years. Nearly all the animal remains of this immeasurable space of time belong to animals that also lived in water, and it seems likely that for the greater part of the time organisms adapted to a life on land did not yet exist.

The algae, then, formed the beginning of vegetable life, and contained the original forms of all other plants, and in the water plants of to-day, both fresh and salt, we can study the representatives of the ancestors of the whole vegetable kingdom. The algae differ very much both in size and in variety of form. There are the minute fresh-water algae which form the green covering of ponds and ditches, and there are gigantic sea algæ which can reach a length of 400 feet. In the Sargasso Sea of the Atlantic Ocean there are algæ forests that cover a space of about 40,000 square miles, and in the primeval ocean there were probably still larger forests that in the opinion of some geologists helped to form our coal seams.

The fungi are closely allied to the simplest algæ. They differ from most other plants in forming no green

colouring matter, or chlorophyll, and in living, like animals, on organic food. They probably arose from the algæ as a result of adaptation to a parasitical form of life.

The lichens which form crust-like coverings on rocks, stones, trunks of trees, etc., are each of them composed of two distinct plants, an alga which produces chlorophyll and builds food material, and a fungus which lives upon it as a parasite. The two plants are so completely interwoven that the lichen appears to be a single organism.

The first stemmed plants were mosses and ferns, the mosses probably forming the transition stage between unstemmed plants and ferns. These formed the predominant vegetation during the period when the coalbearing rocks were being laid down, but a few early conebearing trees were also in existence, these being the earliest plants to produce seeds. They must have arisen from the ferns, and they, with their simple and insignificant flowers, producing naked seeds, were the ancestors of the highest family of the vegetable kingdom, the true flowering plants, with enclosed seeds.

With the exception of the ancient seaweeds that have been mentioned, the fossil history of plants opens when they were already very highly organized, and as various in character as the plants of to-day, though they were of a more primitive type. They must have had an immensely long history behind them, which, as far as direct evidence goes, is a sealed book. This unrecorded history covers undefined but immense epochs during which plants evolved from their one-celled stage and became great and complex organisms. Many groups of animals came into existence, reached their highest development, and became extinct, as we know from the fossil record of the rocks, and still we know very little

of plants. It is not till we come to that period of the earth's history when the carboniferous rocks were formed that we have much information preserved for us, but in them we have a wonderful record of the plants living when they were laid down. Besides many other

beds, they include the coal measures, which are the buried remains of vast swampy forests. These alone reach sometimes a thickness of two miles and must have taken millions of years to form.

Each seam represents the entire existence of a great forest, the trees of which, generation after generation, lived, died, and decayed into vegetable mould, upon which another generation sprang up, until their débris had reached a depth of many feet. Then the plain on which the forest grew began slowly to sink. It became a marsh, and finally the sea rolled over the fallen trees; sand and mud, washed from the higher land by rivers, slowly buried them on the sea-floor, and the shells and skeletons of innumerable generations of ocean life increased the deposits which were making the shallow sea continually shallower. At last it was filled up. Its waters receded, and upon the mud brought down by the rivers a new forest flourished, until it too sank beneath the sea and was buried in mud, sand, and sea-animals'

remains. A hundred distinct seams and the rock layers between them sometimes demonstrate the existence of a hundred different forests on the same site, each in turn overwhelmed by the sea. The immense weight of overlying beds converts the vegetable débris into the structureless, hard, shiny, carbonaceous material we call coal, but in spite of the great change which the plants have in general undergone, the coal measures have also preserved so much which has kept its form at least, or both form and structure, that we have a remarkable knowledge of the plants that existed during the period of their formation. Of the plants of later eras we have much less complete information, until we come to those of the present day.

There are two different types of fossils, produced respectively by incrustation and petrifaction.

In incrustation a plant or portion of a plant is encased in mineral material, which may thus provide a perfect print or cast of its surface. The tissues either decay or are changed by pressure into structureless coal. In the case of a seam of coal a whole forest in all its generations has been encased in mineral material. The mass of decayed vegetation has lost its structure, but the plants on the upper and lower surfaces of the mass have left their imprint on the hardened mud with which they are in contact. Thus we get the beautiful impressions of fern-like fronds with which most people are familiar.

In petrifaction the whole substance of the plant has been impregnated with mineral matter in solution. As a result of evaporation the mineral matter solidifies in the tissues of the plant, the structure of which in this way is often marvellously preserved.

In the English coal measures the most important

examples of petrifaction are contained in the so-called coal balls, which are frequently found both in the actual seams and in the beds between in certain mining districts, especially in Lancashire and Yorkshire. These balls consist of vegetable remains of all kinds impregnated with lime, to which they owe the preservation of

COAL BALL

their minute structure, and embedded in mineral matter of the same kind, so that they resemble masses of stone. They were formed during the periods when the successive forests had sunk beneath the sea, and the lime which has gone to their formation is due mainly to the shells of the sea creatures that lived and died in the waters by which the forest

was submerged. With the help of the fossils preserved in such abundance and perfection we are able to reconstruct the forest plants that flourished in the epoch when the carboniferous rocks were formed. It should be remembered that it is the swamp flora only of which we thus acquire a knowledge; we know nothing of the plants of that era which grew on high ground.

Still, even in those immensely remote ages-and the little we know of the plants of still earlier epochs shows them to have been in the main not very differentnearly as many different types existed as at the present day. All except the very highest groups were represented, but the aspect of the primeval forest in which they grew must have been strangely different from that