developing mentally in response to appropriate stimulation, and (2) on the amount of appropriate stimulation supplied—as is clearly the case in man, in whom beyond all other animals there has been an evolution of this power.
In higher animals therefore the power of individually acquiring reason, of varying, of developing mentally, in response to appropriate stimulation, is strictly analogous to their power of individually acquiring corporal traits, of varying, of developing physically in response to appropriate stimulation. But just as in lower animals, such as the sponges and cœlenterates, the physical structures develop in the absence of all stimulation other than that of sufficient food, so also do such mental traits as they exhibit, reflex action and even instinct, develop in higher animal in the absence of all other stimulation; and therefore the development of the reflexes and instincts is strictly analogous to the development of the physical structures of lower animals; that is, appropriate stimulation does not cause variations and developments in reflexes and instincts, but merely calls pre-existing reflexes and instincts into activity; they develop quite apart from stimulation.
Reflexes and instincts, like the structures of lower animals, and like what I may call the groundwork of many of the structures of higher animals, then, have been developed by Natural Selection to provide reactions against actions invariably occurring in the simple environment; but reason, like that part of the physical development of higher animals which is achieved only as a reaction to stimulation, is achievable only as a reaction to appropriate stimulation acting on the inborn power to vary, to develop mentally; by means of it, as by means of their variable physical structures, higher animals are brought into completer harmony with a variable and complex environment, in which occur
