in of other substances, until through this process of replacement the shell or bone has become changed to stone, or, as it is often termed, petrified; and the older these objects are and the deeper they lie in the rocks the more complete are the changes they have undergone.

So completely, though gradually, do these changes take place, that even the minute structure of wood or bone may be seen under the microscope, the exact shape of each little cell having been retained, although the original material of which it was composed has been replaced by silica, or flint, as it is more familiarly called.

Since these changes require certain favorable conditions, and take place very slowly, only objects that retain their shape for a considerable time can be thus altered; so, flesh can never be turned into stone, and consequently there is no such thing as a "petrified body."

The familiar experiment of soaking a bone in weak muriatic acid shows how much animal matter there is in a skeleton; and it will surprise any one who will try a tooth-that of a horse, for example-in place of a bone, to find how much of this, too, is made up of gristle-like material. There is so much of this in the tusks of elephants or teeth of sperm whales that they can be made into gelatin, and this might be used for making elephant or whale jelly. This is the reason why the tusks of ancient elephants are so rarely found; the soft material of the tusks washed out so much more rapidly than mineral substances filtered in to replace it, that they crumbled to pieces and disappeared. There is another class of fossils in a measure intermediate between the actual preservation of a bone or shell and its mere impression; and this is where such objects as the shell of a crab or the leaf of a tree have been pressed extremely thin, but have left their exact outline in color on the rock, as if painted by the hand of nature. This color-printing is usually due to the presence of iron in the soil in which the object was buried, and its combination with organic matter makes a stain.

But if flesh and animals of soft texture perish completely so far as actual substance is concerned, the impressions such objects made in the sand or mud on which they rested and the casts formed by the mud which settled about or in them may remain; and it is wonderful to find that such delicate creatures as soft-bodied jellyfish, or sea-nettles, have left traces of their former presence even in some of the most ancient rocks. This, of course, could happen only where the water was quiet and soft mud plentiful, so that these delicate animals were buried immediately after death. If we imagine a host of jellyfish resting on the shallows of some quiet little Cambrian bay, and that into this poured a river suddenly made turbid with the mud created by some local inland rain, it will give us some hint as to how their preservation might take place.

The sea to-day swarms with jellyfish, or medusæ, especially in warm latitudes, and so it must have done in past ages; that "fossil jellyfish' should be found only in one or two favored localities shows how rarely just the right conditions for their preservation occurred.

Insects that is, the imprints of insects-have been found by the thousand in the soft, fine-grained shale of Florissant, Colo.; and many others, including such familiar forms as cockroaches and dragon-flies, have been taken from the great storehouse of wonderful fossils, the lithographic stone quarries of Solenhofen, Bavaria. And if traces of such delicate and fragile creatures as these have been thus preserved, it is not surprising to find imprints of feathers and of the tough hides of reptiles, even when not armed with plates and spines of bone; and from these we may learn much as to the covering of these bygone animals.

There is still another class of impressions which furnishes assistance in reading the history of the past, and these are footprints. As children we may have delighted in tales of hunters tracking their game through the forest, or of Indians following the faint trail of fleeing enemies; while still more recently we may have read with equal interest Mr. SetonThompson's stories, and followed the tracks of Wahb or Molly along the margin of the page. In much the same manner the paleontologist patiently follows the trails of long-vanished animals that ages ago passed over the sands of Time and out of existence. For, as the animals of to-day leave their footmarks beside the pond in the meadow, on the sands of the seashore, or along the margin of the river, just so the creatures of the past left their imprints on the sand or in the mud, to harden into stone and bear an indelible record of the life of other days. It is not only the larger animals that left this record in the rocks, but scores of smaller, more insignificant creatures-crabs, shell-fish, and even insects. Many of these marks have been read by comparing them with the impressions left by existing animals as they crawled over mud and sand, or over wet plaster of Paris; but now and then we come upon markings quite different from those made by any animal with which we are acquainted. In such cases, knowing the kind of tracks made by living animals and the manner in which they move their legs, it is necessary to cast about for some fossil form whose feet can be made to fit the impressions, and in this way were interpreted the markings on Cambrian rocks now ascribed to trilobites.

If the tracks of trilobites were puzzling because they were different from those made by any modern animals, those made by the great reptiles called dinosaurs were long misinterpreted for precisely the opposite reason. Ordinarily the feet of different groups of animals are constructed on different plans, so that footprints may show not only whether they were made by mammals, birds, reptiles, or batrachians, but even indicate the

particular division to which the individual making them belonged. But the feet of some dinosaurs were so much like those of birds that for many years the tracks made by them were ascribed to gigantic birds. Close observers, however, pointed out that some of the finest impressions showed that the texture of the skin was quite different from that covering the feet of birds, while in most cases the bones of the toes were shorter and heavier, and subsequent discoveries have made it clear that these footprints are those of dinosaurs.

Very often tracks are all we have to tell that some animals ever existed, for their bones were either destroyed or lie buried deep in the rocks in places now inaccessible. A well-known instance is that of the famous footprints in the red standstone of the Connecticut Valley, which bear testimony of the presence of a host of animals, great and small, but two or three of which have ever come to light. When these tracks were made, a long, narrow bay or estuary ran northward from Long Island Sound, and the rocks tell that at times the shores were left dry to bake in the sun, and again that they were overflowed by water, sweeping down quantities of mud and sand, filling up all impressions, and making casts of the tracks of those creatures that had wandered by the waterside.

How fossils are laboriously gathered and patiently prepared are stories by themselves, but stories that in the present instance may be passed by; what does concern us is the method by which these characters of stone are made to tell the story of the past life of our continent. It is not so many years ago that fossils were looked upon as mere sports of nature,'' interesting from their resemblances in some cases to shells or bones, but having no meaning whatever. A little later their real nature was acknowledged, but they were regarded as "medals of creation," marking various stages in the history of the world, but of importance mainly for the identification of strata and determining the distribution of rocks. Now, however, it is recognized that fossils do not merely mark different epochs in the history of the past, but that only by their aid can we determine the relationships that animals bear to one another, and only through them can we hope to trace the development and distribution of living things.

The student of the past has at his command the teeth and bones of vertebrates, sometimes complete skeletons, their footprints, and, more rarely, imprints of their coverings or even outlines of their forms. Of invertebrates, there are shells or casts of shells, the hard coverings of such creatures as crabs, impressions of soft animals like jellyfishes, and the trails made by these various creatures as they crept over the shore. Fossilized logs and seeds, clean-cut impressions of leaves, rushes, and seaweeds, combine to tell the plant-life of the ancient world, while the rocks in which all these are preserved add their information to that of the fossils. And with the aid of all this material it is possible to picture

plant and animal life as it was at various epochs of the world's history, although these pictures are, of necessity, more or less incomplete and lacking in details.

The story of the past is read as a Chinese book seems to be, from the end backward, and it is necessary to study not only the structure of animals now living, but their appearance and habits, in order to understand the meanings of the fragments of bone from which we must derive our knowledge of the animals that have long ceased to be. Even with a good knowledge of modern animals it is often a difficult matter to tell the relationships, habits, and appearance of many extinct forms, as they were so different from any now living that we have no term of comparison. Still, careful research has done much within the last twenty-five years to increase our knowledge, not merely by discovering new animals, but by the finding of more complete specimens of those already known to us by fragments. Fortunately, too, for the student, while the majority of living animals differ more or less from those of the past, there are a few of the old-fashioned types still remaining to throw some light on those that have passed away.

From the animals and plants we are able to tell what the climate was at different periods; for when, in the rocks of Wyoming, for example, we find fossil palms resembling those now living in the tropics, or a breadfruit tree turns up in California, we naturally infer that the climate of that part of the world was very much warmer then than now. So the former presence in Greenland of forest trees similar to those now growing in New York indicates that the climate of the entire globe was once milder. And if remains of great reptiles are found associated with plants, these inferences are strengthened, for the reptiles of to-day have their headquarters in warm countries, and large forms never cross the line of frost.

On the other hand, the bones of reindeer in southern Europe, and those of musk-ox in Kentucky, tell of a time when these places were far colder than now, and that their tale is true we know from the testimony left by the great ice-sheets that have given their name to the Glacial period. More than this, since the bones are those of species still living, we know that this cold period could not have occurred so very long ago.

Sometimes we may even go a little further than this, and tell what the weather was at some particular time; there are prints of rain-drops, and these may even show the direction of the wind, casts of gaping cracks in the sun-dried mud, telling of long drought, and marks left by the rippling waves as the tide went out, speaking of gentle breezes and fair weather. It is always well to have corroborative evidence in doubtful cases, for if Nature does not exactly play tricks on us, her messages, like those of the Delphic oracle, are occasionally obscure, and capable of being translated in more ways than one.

Cuvier inferred from the bones of the elephant and rhinoceros that at the time they lived Europe rejoiced in a warm climate, but later discoveries showed that these animals were clothed in fur and fitted to endure the cold.

In cases such as these, plants furnish reliable testimony as to climate, for they are less adaptive than animals, or show their adaptation much more plainly. When we go north we find the trees growing smaller and smaller, until finally they disappear; but the reindeer and musk-ox are large animals, and the polar bear and Greenland whale even larger than their tropical relatives.

And if fossils tell what the climate was while the rocks in which they are contained were forming, the rocks, on the other hand, may show why the climate changed, and with it the plant and animal life of that portion of the globe. The upheaval of mountain ranges has cut off warm and moisture-laden winds, transforming verdure-clad plains into arid wastes; the slow rise of great masses of land has cast a chill over vast areas, transforming those species that we know not why-can respond to changed conditions, pushing southward, or blotting out of existence those that can not. The mountain range may countless ages ago have been leveled to a plain, the continent again sunk beneath the sea and again risen, but from the rocks and fossils we may learn the story of these changes, set the former boundaries of the land, and people the earth with its long-vanished life.

If the rocks from two widely separated localities are found to contain the same or even similar species of fossil land animals, it is to be inferred that these rocks were formed at about the same period of time, and that there was a land connection between the two places. These are very general propositions, but in actual use there are several factors to be taken into consideration, and with invertebrates the case is yet more complicated.

If the fossils are very different in their nature, we may be sure that the rocks were separated either by time or space; and if the fossils are those of mammals, they will probably tell which of these two possibilities is a probability. For here it may be said that the different kinds of animals keep as it were different kinds of time, the low animals of simple structure seeming to change much more slowly than those higher in the scale. This is really what might be expected, for the more highly organized a creature the more susceptible should it be to changes of any kind, although another factor probably plays a part here, the fact that the simpler animals as a rule move about less, and live now, and did in the past, under more uniform conditions than their relatives. And among animals the mammals, after they became fairly established, changed the most rapidly of all, so that, aside from the marsupials, there is not now living