He, with the domestic animals and plants which were to be the companions of all his pilgrimages, could wander over the whole earth and choose his home.  Placed at the head of creation, gifted with intellect to make both animals and plants subservient to his destinies, his introduction upon the earth marks the last great division in the history of our planet.  To designate these great divisions in time, I would urge, for the reasons above stated, that the term which is indeed often, though not invariably, applied to them, be exclusively adopted,—­that of the Ages of Nature.

But these Ages are themselves susceptible of subdivisions, which should also be accurately defined.  What is the nature of these subdivisions?  They are all connected with sudden physical changes in the earth’s surface, more or less limited in their action, these changes being themselves related to important alterations in the organic world.  Although I have stated that one general character prevailed during each of the Ages, yet there was nevertheless a constant progressive action running through them all, and at various intervals both the organic and the physical world received a sudden impulse in consequence of marked and violent changes in the earth-crust, bringing up new elevations, while at the same time the existing animal creation was brought to a close, and a new set of beings was introduced.  These changes are not yet accurately defined in America, because the age of her mountains is not known with sufficient accuracy; but their limits have been very extensively traced in Europe, and this coincidence of the various upheavals with the introduction of a new population differing entirely from, the preceding one has been demonstrated so clearly that it may be considered as an ascertained law.  What name, then, is most appropriate for the divisions thus marked by sudden and violent changes?  It seems to me, from their generally accepted meaning, that the word Epoch or Era, both of which have been widely, though indiscriminately, used in geology, is especially applicable here.  In their common use, they imply a condition of things determined by some decisive event.  In speaking of human affairs, we say, “It was an epoch or an era in history,”—­or in a more limited sense, “It was an epoch in the life of such or such a man.”  It at once conveys the idea of an important change connected with or brought about by some striking occurrence.  Such were those divisions in the history of the earth when a violent convulsion in the surface of the globe and a change in its inhabitants ushered in a new aspect of things.