Among plants, as distinguished generally from animals by the production and abundant use of chlorophyll and of cellulose, we have still existing on the Earth a range of forms extending from almost the most primitive organism that we can imagine up to the splendid Seed Plant, specialized in a hundred ways. Every pool, every soil, swarms with bacteria, the lowliest form of life--organisms exceedingly minute, exceedingly simple, and capable of existing under highly diverse conditions both physical and chemical. Thence we can trace an irregular ascending scale through the Fungi, the Algæ, Mosses, Horsetails, Ferns, and Club-mosses, to the Conifers, and on to the highest of the Seed Plants, which exceed in their beauty of structure and complicated life anything that has gone before them. In fact, as Theophrastus says, your plant is a thing various and manifold. And this existing vegetation with its thousand forms is but the present manifestation of the vital activity which has populated the earth during tens of millions of years. The oldest rocks which have been preserved to us in such a condition as to yield remains of plants and animals in a recognizable form are those known as Cambrian, the deposition of which occurred at a period which geologists have variously calculated as from, say, 20 to 100 or more millions of years ago. Yet even at that immensely remote period, life, both vegetable and animal, was already abundant and diverse, as well as highly organized. As Darwin long ago pointed out, the geological record does not go back nearly far enough to allow us any insight into the evolution of the earlier forms of life. Below the Cambrian rocks, as represented in these islands and in Europe generally, with their well-developed fauna, are tens of thousands of feet of strata which once, no doubt, were sediments at the bottom of the sea, and later on hardened into slates and sandstones in which were embedded remains of more primitive organisms; but these rocks have been so altered during the immense period of their existence by heat and pressure and the other vicissitudes to which the restless crust of the earth is subject that they now present a mass of granite-like material in which all trace of organic life has been destroyed. In America the rocks of corresponding age are better preserved, and have yielded a limited fauna displaying an already advanced stage of evolution. To account for the strange paucity of animal remains it has been suggested that the creatures of these earliest times were soft-bodied, so that after death they left no trace behind. It may be noted that the pre-Cambrian rocks contain beds of limestone and of carbon (in the form of graphite); such beds, in later rocks, are composed of organic materials, the limestones being formed of the skeletons of minute marine creatures, particularly Foraminifera, and the carbon deposits of the remains of plants.