The earliest stages of human progress are very slow, and much interrupted by wars of conquest and extermination. We find no difficulty, therefore, in assigning a great antiquity to some of these ruined cities, and a still greater antiquity to the curious mounds and earthworks in the valley of the Mississippi, without necessarily inferring that these are the remains of any civilisation superior to what history has made known to us. And before these mounds were constructed, there might have passed a long epoch in which man wandered wild by the rivers and in the forests of this continent. This last-mentioned epoch of mere savage existence, some of our speculative philosophers would extend to an enormous duration. We are not disposed, by any evidence yet submitted to us, to expand this period to what we must not call a _disproportionate_ length, because we have not the whole life of the human race before us; but which, arguing on those progressive tendencies which, notwithstanding the impediments and checks they receive, constitute the main characteristic of the species, seems an _improbable_ length. Let the geologist, however, to whom this part of the problem must be handed over, pursue his researches, and we need not say we shall be happy to receive whatever knowledge of the now forgotten past he can bring to light. CAXTONIANA: A SERIES OF ESSAYS ON LIFE, LITERATURE, AND MANNERS. By the Author of ‘The Caxton Family.’ PART XVI. NO. XXII.—ON CERTAIN PRINCIPLES OF ART IN WORKS OF IMAGINATION.