From a study of all these various relics of the past--human, animal, and vegetable--in connection with geologic changes, the student seeks to estimate approximately the date at which man first appeared upon the earth. He observes the slow accumulation of surface deposits and speculates on the time requisite to bury the works of man hundreds of feet deep in dilluvium. He studies savagism in its different phases as portrayed in a previous volume; notes how tenaciously the primitive man clings to old customs, how averse he is to change and improvement; and then reflects upon the centuries that would probably suffice for beings only a little above the beast to pass successively from the use of the shapeless stone and club to the polished stone spear and arrow and knife, to the partial displacement of stone by the fragment of crude metal, to the smelting of the less refractory ores and the mixture of metals to form bronze, and to a final triumph in the use of iron. He reflects farther that all this slow process of development precedes in nearly every part of the world the historic period; that its relics are found in the alluvial plains of the Nile, buried far below the monuments of Egyptian civilization, a civilization, moreover, which dates back at least two thousand years before Christ. Searching the peat-beds of Denmark, he brings to light fossil Scotch firs in the lower strata mingled with relics of the stone age; oak-trees above with implements of bronze; and beech-trunks in the upper deposits, corresponding with the iron age and also with the present forest-growth of the country. He tries to fix upon a period of years adequate to effect two complete changes in Danish forest-trees, bringing to his aid the fact that about the Christian era the Romans found that country covered as now with a luxurious growth of beech, and that consequently eighteen hundred years have wrought no change. Having thus established in his mind the epoch to which he must be carried by the relics of the alluvial deposits, he remarks that during all this period climate has not essentially changed, for the animal remains thus far discovered are all of species still existing in the same climatic zone.