to connect the subject with existing knowledge was made in the use of the comparison of vegetable with animal life, a comparison which Aristotle had employed with small success. Cesalpino, provided with more botanical and zoological knowledge, endeavoured to arrive at more definite ideas of the movement of the nutrient juices in plants, and when Harvey discovered the circulation of the blood in the beginning of the 17th century, the idea at once arose that there might be a similar circulation of the sap in plants. Thus a first hypothesis, a definite question was framed, and attempts were made to decide it by more exact observation of the ordinary phenomena of vegetation, and still better by experiment; and though a discussion which lasted nearly a hundred years led to the opinion that there is no circulation of sap in plants corresponding to the circulation of blood in animals, the result was obtained by the aid of this hypothesis derived from a comparison between animals and plants. The important discovery that leaves play a considerable part in the nourishment of plants, was to some extent an incidental product of the investigation of the former question, and it preceded that of the decomposition of carbon dioxide by the green parts of plants by more than a hundred years. To give another example; it was obviously a comparison of certain phenomena in vegetable life with the propagation of animals which paved the way for the discovery of sexuality in plants; long before Rudolf Jacob Camerarius made his decisive experiments (1691-1694) on the necessary co-operation of the pollen in the production of seeds capable of germination, the idea had been entertained that there might be an arrangement in plants corresponding to the sexual relation in animals, though that idea was highly indistinct and distorted by various prepossessions. In like manner the interest excited by the discovery of the irritability of the Mimosae in the 17th century, and of similar phenomena of movement in plants at a later time, was mainly due to the striking resemblance suggested between animals and plants; and the first researches into the subject were obviously intended to answer the question whether the movements in plants are due to conditions of organisation similar to those in animals. In all cases of this kind it was matter of indifference whether the analogies presupposed were finally confirmed after prolonged investigation, as in the question of sexuality, or disproved as in that of the circulation of the sap. The result was of less importance than the obtaining points of departure for the investigation. It answered this purpose to adopt certain actual or only apparent analogies between plants and animals, and to assume, to some extent to invent, certain functions for the apparently inactive organs of plants, and to interrogate them upon the point. Scientific activity was set in motion, and it mattered not what the result might be. In