We find universally distributed in nature the sensation of the mechanical stimulus of gravitation, the most comprehensive statement of which is given in Newton's law of gravity. According to this fundamental and all-ruling law, any two particles of matter are attracted in direct proportion to their mass and inverse proportion to the square of their distance. This form of attraction, also, can be traced to a "sensation of matter" in the mutually attracting atoms. The local sensation that any body provokes by contact with the surface of an organism is felt as pressure (_baros_). A stimulus that causes this pressure alone brings about a counter-pressure as a reaction, and an effort to neutralize it, the pressure-movement (_barotaxis_ or _barotropism_). Sensitiveness to pressure or the contact of solid bodies is found throughout the organic world; it can be proved experimentally among the protists as well as the histona. Special sense-organs have been developed in the skin of the higher animals as the instruments of this pressure-sense (baræsthesis) in the form of tactile corpuscles; they are most numerous at the finger-tips and other particularly sensitive parts. In many of the higher animals there is a fine sense of touch in the feelers or tentacles, or (in the higher articulates) in the horns or antennæ. Moreover, these tactile and prehensile organs are also very widely found among the higher plants, especially the climbing plants (vines, bryony, etc.). Their slender creepers, which roll out spirally, have a very delicate feeling for the nature of the supports which they embrace; they distinguish between smooth and rough, thick and thin supports, and prefer the latter. Many of the higher plants, which are particularly sensitive to pressure, have, to an extent, special organs of touch (tentacles), and reveal this by the movements of their leaves (the sensitive plants, _mimosa_, _dionæa_, _oxalis_). But even among the unicellular protists we find that the contact of solid bodies has an irritating effect, the perception of which provokes corresponding movements (_thigmotaxis_ or _thigmotropismus_). A peculiar form of pressure-sensation is produced in many organisms by the flow of liquids; in the mycetozoa, for instance, it provokes counter-movements (_rheotaxis_, _rheotropismus_), as Ernst Strahl showed by his experiments on _æthelium septicum_. We have an interesting analogy to the thigmotaxis of the viscous living plasm in the elasticity of solid inorganic bodies, such as an elastic steel-rod. In virtue of its springy nature, the elastic rod reacts on the pressure of force that has bent it, and endeavors to regain its former position. The spiral spring sets the works of the clock in motion in virtue of its elasticity.