As has been stated on a former page, the evidence points to life having originated in the water, at a period extremely remote. The most lowly as well as the most minute of all organisms are the bacteria, some of which are in size beyond the limit of the most powerful microscope to detect, their presence being known only by their chemical actions. The most primitive groups of bacteria, known as prototrophic, are able to live without light, deriving their nourishment by the breaking up of inorganic chemical compounds. It is difficult to conceive of any living organism more primitive than these, and quite possibly they recall that dim borderland where merely chemical structure and action mysteriously advanced into the cell structure and purposive chemical changes which we call life. From that lowly stage the evolution of plant life has been marked especially by three great forward bounds, of inestimable importance. The first of these was the “invention” of chlorophyll, which allowed plants to use for their life-processes the vast supply of energy furnished by the Sun. Sunlight then became essential to life, and the Algæ, the probable ancestors of all the higher plants, were developed, presumably through the peculiar _Cyanophyceæ_, or “Blue-green Algæ,” in which the chlorophyll is in a somewhat undifferentiated condition. Much later than this stage, yet far back in the history of evolution, occurred the second of the great forward steps. This was the desertion of the water for the land, which opened up for the plant world vast new fields and a great variety of new conditions. The final stage was reached by the abandonment of the aquatic mode of pollination by means of swimming spermatozoids, as still found in the Maidenhair Tree (_Ginkgo_), Cycads, Ferns, and groups lower in the scale, and the adoption instead of pollination through the medium of the air, “which” (to quote Mrs. Arber’s happy phrase) “has won for them the freedom of the land.” The Seed Plants, then, achieved their wonderful abundance and variety owing to the highly stimulating conditions offered by a terrestrial existence; we must assign to all the existing types a long terrestrial ancestry. How, then, about the water plants whose leaves and flowers so decorate our lakes? There seems no doubt[13] that they are species which have left the land to resume the aquatic habits of their remote ancestors. With few exceptions they retain the aerial mode of pollination which is the pride of the specialized land plants. The pressure of competition has probably driven them into the water, where they descend as far as the lessening light-supply will allow. Some--presumably the earliest to take to an aquatic life--have all their relations to keep them company, the remote ancestor which adopted an aquatic habit being now represented by many species, or even by many genera. In other cases a terrestrial genus or order has few or only a single aquatic representative. It may be assumed that in such a case the aquatic habit has been recently acquired. The great majority of water plants send their flowers up above the surface to be pollinated by wind or (more rarely) by insects. It may be noted that few of the more highly evolved groups of Seed Plants are represented in the aquatic flora; wind-pollinated flowers of a rather primitive type of structure are the rule in our lakes and rivers; which points to an early assumption of the aquatic habit, and suggests that the land is more favourable than the water for the evolution of higher types.