It must explain clearly and definitely, and not by vague generalities or superficial analogies, why, though mankind started presumably with the same capacities and at the same time, there now exist such wide differences in social development. It must account for the arrested civilizations and for the decayed and destroyed civilizations; for the general facts as to the rise of civilization, and for the petrifying or enervating force which the progress of civilization has heretofore always evolved. It must account for retrogression a well as for progression; for the differences in general character between Asiatic and European civilizations; for the difference between classical and modern civilizations; for the different rates at which progress goes on; and for those bursts, and starts, and halts of progress which are so marked as minor phenomena. And, thus, it must show us what are the essential conditions of progress, and what social adjustments advance and what retard it. It is not difficult to discover such a law. We have but to look and we may see it. I do not pretend to give it scientific precision, but merely to point it out. The incentives to progress are the desires inherent in human nature--the desire to gratify the wants of the animal nature, the wants of the intellectual nature, and the wants of the sympathetic nature; the desire to be, to know, and to do--desires that short of infinity can never be satisfied, as they grow by what they feed on. Mind is the instrument by which man advances, and by which each advance is secured and made the vantage ground for new advances. Though he may not by taking thought add a cubit to his stature, man may by taking thought extend his knowledge of the universe and his power over it, in what, so far as we can see, is an infinite degree. The narrow span of human life allows the individual to go but a short distance, but though each generation may do but little, yet generations, succeeding to the gain of their predecessors, may gradually elevate the status of mankind, as coral polyps, building one generation upon the work of the other, gradually elevate themselves from the bottom of the sea. Mental power is, therefore, the motor of progress, and men tend to advance in proportion to the mental power expended in progression--the mental power which is devoted to the extension of knowledge, the improvement of methods, and the betterment of social conditions. Now mental power is a fixed quantity--that is to say, there is a limit to the work a man can do with his mind, as there is to the work he can do with his body; therefore, the mental power which can be devoted to progress is only what is left after what is required for non-progressive purposes.