Lichens afford a very curious instance of the association of vegetable and animal functions in the same plant. They are really formed of two distinct organisms: a body which is a low form of Alga or sea-weed, and a parasitic form of fungus, which lives upon it. The former has a plant life, living on inorganic matter and forming the green cells, or chlorophyll, which are the essential property of plants, enabling them under the action of the sun’s rays to decompose carbonic dioxide; while the parasite lives like an animal on the formed protoplasm of the parent stem, forming threads of colourless cells which envelop and interlace with the original lichen of which they constitute the principal mass, as in a tree overgrown with ivy. Even in existing and highly developed plants we find some curious instances of reversion towards animal life. Certain plants, for instance, like the Dionæa or Venus’ fly-trap, finding it difficult to obtain the requisite supply of nitrogenous food in a fluid state from the arid or marshy soil in which they grow, have acquired a habit of supplying the deficiency by taking to an animal diet and eating flies. Conjoined with this is a more highly developed sensitiveness, and power of what appears to be voluntary motion, and a faculty of secreting a sort of gastric juice in which the flies are digested. The fundamental property also of decomposing carbonic dioxide and exhaling oxygen depends on light stimulating a peculiar chemical action of the chlorophyll, and at night leaves breathe like lungs, exhaling not oxygen, but the carbonic dioxide. The records of geology, imperfect as they are, show a continued progression from these simple and neutral organisms to higher and more differentiated forms, both in the animal and vegetable worlds. These records are imperfect because the soft bodies of the simpler and for the most part microscopic forms of protoplasm and cell life are not capable of being preserved in petrifactions, and it is only when they happen to have secreted shells or skeletons that we have a chance of identifying them. Still we have a sufficient number of remains in the different geological strata to enable us to trace development. Thus, in the vegetable world, in the earliest strata, the Laurentian, Cambrian, and Silurian, forming the primordial period, which forms a thickness of some 70,000 feet of the earth’s crust—or more than that of the whole of the subsequent strata, Primary, Secondary, Tertiary, and Quaternary, taken together—we find only vegetable remains of the lowest group of plants, that of the Tangles or Algæ, which live in water. Forests of these sea-weeds, like those of the Aleutian Islands, in some of which single tangles stream to the length of sixty feet, and floating masses, like those of the Sargasso Sea, appear to have constituted the sole vegetation of these primæval periods.