Or let us consider the case of an organism which has originated in the shade. Heat, as we know, increases chemical change, in inorganic as well as organic matter; it hastens the disintegration of certain compounds, and alone renders it possible in many cases. In general, we may assert that increase of temperature within certain limits increases assimilation; that is, capacity to assimilate. Therefore, if an animal is placed in the sun, its capacity, that is, its need, to assimilate is increased, although assimilation is much more energetic than before. Need to assimilate or hunger is, therefore, dependent upon the supply of food, although, doubtless, also on other conditions, especially those of light and temperature. If this is true, the hunger of a simple organism that assimilates energetically must be more intense than that of one which assimilates slowly, in spite of the consumption of an enormous quantity of food in the case of the former. Botanists know (Sachs, "Lehrbuch der Botanik," p. 613) "that growth may be so hastened by too high a temperature that assimilation (especially under scanty light) does not suffice to provide the necessary material for it. The transpiration of the leaves may be so increased that the roots cannot repair the loss. And on the other hand, a too low temperature of the soil may so diminish the action of the roots that even a small loss by transpiration cannot be repaired." At what stage of organization psychical hunger is added to mechanical hunger, or whether it may be identified with it, we cannot say. In any case, the former appears exceedingly early, for excitations of hunger may be observed in creatures very low in the scale of being. Certainly hunger is never absent where there is movement. Hunger, a sense of pain, is, therefore, the first impulse to action.[58]