In the gradual growth of thought which leads to ever higher forms of society, the physiologist has very important aid to render. It is his part to show how the two great forces of Habit and Heredity are the powerful physiological factors in the growth or degeneracy of the human race. In these two great facts--viz., the ability to form habits and the power of transmitting the tendencies produced by habits--the mind and body are inseparably blended, and through them a nation becomes chaste or unchaste. Habit can so change the nature as to make what was difficult easy; it can so strengthen the tendencies in directly opposite directions as to both govern, and to a great extent change, the action of the physical organization itself, and the fact of heredity will transmit these changed tendencies to succeeding generations. It is impossible in the long-run to ignore these two facts which so powerfully govern sexual passion, because Nature has established them. Short-sighted views may exist as to the trivial character of the relations prevailing between the sexes. It may be considered of slight importance whether lust or love rule these relations. The slow or remote nature of the evils produced by the violation of Nature’s laws, and the apparent escape of some offenders from immediate penalty, confuse the short sight of the irreligious. But Nature disregards our short-sightedness, sweeps away our theories and self-indulgence, and inexorably avenges the violation of law by gradual but inevitable degeneration of the race. The power which habit exercises over human nature depends upon the physiological character of the nervous system itself, through which our will and thought act. It has been well said by Michel Lévy that periodicity is the law of the nervous system.[4] It is a law which both regulates its physiological action and controls the course of its diseases. Impressions made upon the brain by external objects or by internal sensations modify the condition of the brain. This modification is slight at first, but increases by repetition. When an impression is first made upon the brain, it has to overcome the inertia or unaccustomed state of the organization to receive that kind of impression. But with each repetition this resistance diminishes and a habit is formed. Owing to the rule of periodicity which governs the nervous system, the brain tends to repeat the change which it has once experienced, to recall sensations, and solicit a repetition of changes which have been frequently impressed upon it. Passing impressions may produce little effect in changing the condition of the brain, but when such impressions are often repeated and prolonged, when the attention is fixed upon them and the will engaged in recalling them, then the nervous system itself undergoes modification, and a new disposition of the organization itself is acquired from the continuation and frequent repetition of the same impressions.