We have seen that the physiologist studies something very different from that which the embryologist or naturalist studies. The former investigates a _part_ of the animal, arbitrarily detached from the whole because the complexity of the functions of the simplest organism is such that all of them cannot be examined at once. He adopts the methods of physical chemistry in his investigation and whatever results he obtains are necessarily of the same order. Inevitably, from the mere nature of his method, he can see, in the organism, only physico-chemical phenomena. The embryologist, on the other hand, studies the organism as a whole and seeks to determine how definite forms are produced, and how a change in the external conditions affects the assumption of these forms. We have seen with what little success the attempts to relate embryological processes with physico-chemical ones alone have met. In all studies of organic form mechanism has failed. It is useless to attempt to press the analogies of crystalline form, and the forms assumed in nature by dynamical geological agencies. If the reader examines these analogies critically he will see that they are superficial only. We seem, however, to see in those actions of the organism which are called “tropistic” or “tactic,” reactions of a purely physico-chemical nature, and starting with these as a basis a plausible theory of organic movements on a strictly mechanistic basis might be built up.[21] A “tropism” is the movement of a fixed organism with respect to a definitely directed external stimulus. This movement may be that produced by growth of its parts, or by the differential contraction or expansion of its parts. A “taxis” we may call the motion of a freely-moving organism in response to the same directed stimuli. The movements whereby a green plant turns towards the light are called heliotropic, and those of its roots in the perpendicular direction are called geotropic. The motion of the freely-moving larva of a barnacle, for instance, in swimming towards a source of light are called “phototactic.” [21] Many of Jacques Loeb’s remarkable investigations point in this direction. In all these cases we have to think of the stimulus as a “field of energy” in the sense in which physicists speak of electric, or magnetic, or electromagnetic, or thermal, or gravity fields. In all these cases the factors affecting the movements of the organism are directed ones. An electric field, for instance, (1), is produced by placing the electrodes of a galvanic cell at opposite extremities of a water-trough: we imagine the electrons moving from one side of the trough to the other in parallel lines, and in a certain direction. A light field (2) would be produced by the radiation of light travelling in straight lines through the water. [Illustration: FIG. 18.]