This law rests on two kinds of proof: the one that of experience, and the other that of reason. The former consists of an enumeration of the many instances in which the law verifies itself. Some of these I have already presented to you, and others will soon follow. One may assert, moreover, that no case is known to which the law is not applicable; it is strictly so to those hitherto examined, and not merely in a general way, but in detail; not only to the growth and extinction of great schools, but again to all the variations and oscillations to which art is subject. The second order of proof consists in showing this dependence to be not only rigorous in point of fact, but, again, that it is so through necessity. We will accordingly analyze what we have called the general state of the mind and surrounding circumstances; we shall seek, according to the ordinary standard of human nature the effects which a like state must produce on the public, on artists, and consequently on works of art. Hence we draw a forced connection and a definite concordance, and we establish a necessary harmony which we had observed as simply fortuitous. The second proof _demonstrates_ what the first had averred. II. In order to make this harmony apparent let us resume a comparison already of service to us, that between a plant and a work of art, and note the circumstances in which a plant, or a species of plant, say the orange, may be developed and propagated in a certain soil. Let us suppose all kinds of grain and seed borne by the wind and sown at random; on what conditions can those of the Lorange germinate, become trees, blossom, yield fruit, spread, and cover the ground with a numerous family?