The dependence of animals upon plants for the food by means of which they continue to inhabit the earth, which was pointed out on a previous page (75), shows that the plant world is older than the animal world; but the immense age of both can be appreciated only by a study of stratigraphical geology. The tens of thousands of feet of sedimentary rocks, laid down in slow succession on the floors of ancient seas and lakes, and still reposing layer upon layer, and no less the great gaps in the series produced when, raised into the air, deposition ceased, and thousands of feet of rock were slowly worn away and washed down again into the sea by the action of frost and wind and water, point to periods incalculably remote as measured by the standards which we apply to human history. A few thousands of years measures the span which separates us from the Neolithic Period; but to the geologist a million years is but a convenient unit for expressing, so far as any expression by our time-standards is possible, the huge periods with which he has to deal. And even when we get back as far as the oldest fossils will take us, we are still a long way from having reached the epoch when life on the earth originated. As we work backward and study the fossils of older and older rocks, the multitudinous assembly of plants and animals which fill the world to-day are replaced by other and more primitive forms, many groups approaching each other and merging in common ancestral types. But still, the very oldest fossil-bearing strata contain the remains of organisms already far up the ladder of evolution. The Lamp Shells (Brachiopods), Pteropods, Trilobites, and Worms of the ancient Cambrian rocks have clearly a long ancestral history. Plants are not so abundantly preserved in the rocks as the skeletons and shells of animals, on account of their softer nature; but in the oldest known plants it is again clear that we are dealing with forms by no means primordial. It is the more interesting, then, to note that many very lowly forms of life have come down to us from times immensely remote, and are still present on the earth in abundance, swarming in every sea and in every pond, or nestling in damp crevices of the land; while higher types of immense antiquity still mingle with the crowd of recent Seed Plants, some of them forming noble forest trees. Of especial interest, taking into account the wide distinction which exists between the higher animals on the one hand and the higher plants on the other, is it to find that there are still in existence organisms which are so much on the border-line between these two great groups of living things that they can be referred to one or other only with hesitation, clearly indicating that animal and vegetable life sprang from a common source. Take the group known as Mycetozoa or Myxomycetes. These names alone show the divergent views which men of science have held regarding them,