In animals possessed of a nervous system, contractions only follow stimuli, which are carried to the contractile elements by nervous threads, the internal energy representing the external stimulus being called nervous energy or neurism. But where a nervous system does not exist, as is the case in some low animals and in all plants, external stimuli must be justly supposed to be converted into the same form of energy, which in such organisms has a general circulation throughout the contractile protoplasm. The attainment of some position, favorable for the procurement of relief from some unpleasant sensation, or the acquisition of some agreeable one, or for both, is the important thing directly subserved by such movements in the generality of animals. While we have the best of reasons for believing this to be true in the vast majority of animals, because fundamentally their structure is similar to our own, yet the inference that the same is true of the lowest forms of life is justifiable until it is proved to be mistaken.
Whatever be the nature of any movement, whether the projecting of portions of its own body-substance as pseudopodia in the primitive animal, the movement of flagella or cilia in more specialized forms, or the turning of the radicle of a plant-seedling in overcoming some obstacle, there is no resisting the conclusion that the functions of these organs, when once called into existence, are due to stimuli not unlike those which affect the motions of the limbs of the higher animals, and that the preliminary to all such movements, which are not automatic, is an effort. And as no adaptive movement is automatic the first time it is performed, effort, therefore, may be regarded as the immediate source of all movement. Now, effort is a conscious state, and implies a sense of resistance to be overcome. But when an act is performed without effort, resistance has been overcome, and the mechanism requisite for its performance has been completed. Automatism has now been reached. New movements, in their incipiency, necessarily meet with resistance. How this resistance is overcome, there seems to be some diversity of opinion among physiologists and metaphysicians, but it is generally believed that some such mental state as a sensation or a desire, which may or may not stimulate a natural process as an intervening element in the circuit, is concerned in its subduement. That sense-perceptions are stimuli to the immediate appearance of structural changes or movements is shown by the production of color-changes in animals through changes in the condition of the organs of sight and in the bending of the radicle of a seedling-plant a short distance above its tip in obedience to a communication from the tip of a sensation of hardness, caused by contact with a stone experienced in its downward progress in the ground.