In Geology, which is Natural History extended over all ages, this principle has a still wider scope,—embracing not merely the characteristics and conditions of the beings which now exist, but of all, so far as we can learn regarding them, which have ever existed, and involving the consideration of not merely their peculiarities as races placed before us without relation to time, but also of the history of their rise, increase, decline, and extinction. In studying the _biography_, if I may so express myself, of an individual animal, we have to acquaint ourselves with the circumstances in which nature has placed it,—its adaptation to these, both in structure and instinct,—the points of resemblance which it presents to the individuals of other races and families, and the laws which determine its terms of development, vigorous existence, and decay. And all Natural History, when restricted to the passing _now_ of the world’s annals, is simply a congeries of biographies. It is when we extend our view into the geological field that it passes from _biography_ into _history proper_, and that we have to rise from the consideration of the birth and death of individuals, which, in all mere biographies, form the great terminal events that constitute beginning and end, to a survey of the birth and death of races, and the elevation or degradation of dynasties and sub-kingdoms. We learn from human history that nations are as certainly mortal as men. They enjoy a greatly longer term of existence, but they die at last: Rollin’s History of Ancient Nations is a history of the dead. And we are taught by geological history, in like manner, that _species_ are as mortal as individuals and nations, and that even genera and families become extinct. There is no _man_ upon earth at the present moment whose age greatly exceeds an hundred years;—there is no _nation_ now upon earth (if we perhaps except the long-lived Chinese) that also flourished three thousand years ago;—there is no _species_ now living upon earth that dates beyond the times of the Tertiary deposits. All bear the stamp of death,—individuals,—nations,—species; and we may scarce less safely predicate, looking upon the past, that it is appointed for nations and species to die, than that it “is appointed for _man_ once to die.” Even our own species, _as now constituted_,—with instincts that conform to the original injunction, “increase and multiply,” and that, in consequence, “marry and are given in marriage,”—shall one day cease to exist: a fact not less in accordance with beliefs inseparable from the faith of the Christian, than with the widely-founded experience of the geologist. Now, it is scarce possible for the human mind to become acquainted with the fact, that at certain periods species began to exist and then, after the lapse of untold ages, ceased to be, without inquiring whether, from the “conditions of existence, commonly termed final causes,” we cannot deduce