_General remarks._--In the preliminary chapters of "The Principles of Geology," in the first and subsequent editions, I have considered the question, how far the changes of the earth's crust in past times confirm or invalidate the popular hypothesis of a gradual improvement in the habitable condition of the planet, accompanied by a contemporaneous development and progression in organic life. It had long been a favourite theory, that in the earlier ages to which we can carry back our geological researches, the earth was shaken by more frequent and terrible earthquakes than now, and that there was no certainty nor stability in the order of the natural world. A few sea-weeds and zoophytes, or plants and animals of the simplest organization, were alone capable of existing in a state of things so unfixed and unstable. But in proportion as the conditions of existence improved, and great convulsions and catastrophes became rarer and more partial, flowering plants were added to the cryptogamic class, and by the introduction of more and more perfect species, a varied and complex flora was at last established. In like manner, in the animal kingdom, the zoophyte, the brachiopod, the cephalopod, the fish, the reptile, the bird, and the warm-blooded quadruped made their entrance into the earth, one after the other, until finally, after the close of the tertiary period, came the quadrumanous mammalia, most nearly resembling man in outward form and internal structure, and followed soon afterwards, if not accompanied at first, by the human race itself. The objections which, in 1830, I urged against this doctrine[xvii-A], in so far as relates to the passage of the earth from a chaotic to a more settled condition, have since been embraced by a large and steadily increasing school of geologists; and in reference to the animate world, it will be seen, on comparing the present state of our knowledge with that which we possessed twenty years ago, how fully I was justified in declaring the insufficiency of the data on which such bold generalizations, respecting progressive development, were based. Speaking of the absence, from the tertiary formations, of fossil Quadrumana, I observed, in 1830, that "we had no right to expect to have detected any remains of tribes which live in trees, until we knew more of those quadrupeds which frequent marshes, rivers, and the borders of lakes, such being usually first met with in a fossil state."[xvii-B] I also added, "if we are led to infer, from the presence of crocodiles and turtles in the London clay, and from the cocoa-nuts and spices found in the isle of Sheppey, that at the period when our older tertiary strata were formed, the climate was hot enough for the Quadrumana, we nevertheless could not hope to discover any of their skeletons, until we had made considerable progress in ascertaining what were the contemporary Pachydermata; and not one of these has been discovered as yet in any strata of this epoch in England."