The mechanical theory of the universe, in the ordinary sense, which deals with the mathematical formulation of the laws of planetary movement, had been the work of the seventeenth century. The eighteenth century had seen attempts to explain the _status quo_ of the planetary system as a resultant from the evolution of an elementary molecular state of the cosmic mass. With the close of the eighteenth century there appeared a group of new sciences dealing with subtler energies of matter,--with electricity, galvanism, and above all with the connexions of chemical, electric, and magnetic science. The ideas thus suggested--embraced with some generality under the title Polarity--threw light backward upon the old mechanical conceptions, and gave them a decidedly dynamic character. Even the tranquil rest of geometrical figures came to be explained as a meeting point and transition moment of opposite forces. But these ideas produced an even greater effect on biology. Here, too, the need of a special 'vital force' to explain life and organisation disappeared: organism was but a higher stage, a completer truth of mechanism: and both found their explanation in the antithesis and synthesis of forces, or in differentiation and integration of what has recently been termed an 'idée-force.' In this direction, so far as Schelling was concerned, the obvious stimulus came from the programme sketched by Kielmeyer at Stuttgart in 1793, in a lecture 'on the proportions of organic forces,' According to Kielmeyer there are three types of force in the animal organisation, sensibility, irritability and reproduction[5]. The last of these is the basic force which builds up and propagates the animal system. With irritability, or contraction in response to external stimuli--material adaptation to environment--a higher level of animal life is reached. But the highest of all forces in the living being is sensibility. In this same order may we reasonably conceive that the 'plan of nature' proceeds. Her first products show little beyond that reproductive power which makes broad and high the pyramid of life. But as the creature acquires increasing heterogeneity and a comparatively independent position, it plays the part of a re-agent against stimuli, and a source of movements. Lastly, it not merely responds to, but assimilates and appropriates the impression into a sensation: it internalises the external, and carries within itself by means of the sensibility an ever-increasing picture of the world around it.