of warmth in soil; from which is seen to result an unusual development of that part of the plant to which growth is especially directed at the time. When the warmth of an incubator is increased, the animal organ especially engaged in development at the time is affected in like manner. So that we may assume that the organism is capable of responding to every demand that nature makes upon it under normal conditions; and since the greatest possible assimilation under the existing conditions is thus removed from the control of the creature, the latter appears practically insatiable. This insatiability must appear to the observer an inner impulse of the organism, an effort towards increase of nourishment. It may be called mechanical hunger in distinction from psychical hunger, of which it is the basis. It is not necessary to take into consideration, in the question as to the degree of assimilation possible, the amount of excretion of substance by the organism; we must, on the contrary, assert that this is dependent upon the amount of assimilation. The measure of growth depends, therefore, on the degree of assimilation of new material. This degree, however, like the degree to which the matter may be dissolved in a liquid in the case of inorganic matter, is especially affected by light and warmth. The creature which comes into existence in the sun will experience a decrease of organic change when placed in the shade; and the creature which comes into existence in the shade will experience an increase of such change under the influence of the sun, a decrease again with a return to the shade. This decrease means hunger,--harm. Experiments with zooespores throw an interesting light upon these relations. They show that the zooespores, although suited to very different degrees of light, all shun darkness. Although when in the light they soon come to rest, divide, and copulate, they remain, in the darkness, in a state of continual unrest and motion. They grow so thin "that they almost excite pity" (Strassburger), and finally perish of hunger. Only such zooespores as are distinguished by sex and copulate come to rest, or those of such sorts as prey upon others. It is easy to perceive that the unrest of the zooespores in the darkness springs from lack of nourishment, from hunger; they seek feverishly for the light, without which assimilation follows with insufficient energy to satisfy need and render life possible. In darkness, copulation alone can do this; copulation takes, then, the place of normal nourishment.