The quantitative value of animated life, as a geological agency, seems to be inversely as the volume of the individual organism; for nature supplies by numbers what is wanting in the bulk of the animal out of whose remains or structures she forms strata covering whole provinces, and builds up from the depths of the sea large islands, if not continents. There are, it is true, near the mouths of the great Siberian rivers which empty themselves into the Polar Sea, drift islands composed, in an incredibly large proportion, of the bones and tusks of elephants, mastodons, and other huge pachyderms, and many extensive caves in various parts of the world are half filled with the skeletons of quadrupeds, sometimes lying loose in the earth, sometimes cemented together into an osseous breccia by a calcareous deposit or other binding material. These remains of large animals, though found in comparatively late formations, generally belong to extinct species, and their modern congeners or representatives do not exist in sufficient numbers to be of sensible importance in geology or in geography by the mere mass of their skeletons. [Footnote: Could the bones and other relics of the domestic quadrupeds destroyed by disease or slaughtered for human use in civilized countries be collected into large deposits, as obscure causes have gathered together those of extinct animals, they would soon form aggregations which might almost be called mountains. There were in the United States, in 1870, as we shall see hereafter, nearly one hundred millions of horses, black cattle, sheep, and swine. There are great numbers of all the same animals in the British American Provinces and in Mexico, and there are large herds of wild horses on the plains, and of tamed among the independent Indian tribes of North America. It would perhaps not be extravagant to suppose that all these cattle may amount to two thirds as many as those of the United States, and thus we have in North America a total of 160,000,000 domestic quadrupeds belonging to species introduced by European colonization, besides dogs, cats, and other four-footed household pets and pests, also of foreign origin.