Fresh water is almost the oldest thing on earth. While the rocks have been melted, the sea growing salter, and the birds and beasts perfecting themselves or degenerating, the fresh water has been always the same, without change or shadow of turning. So we find in it creatures which are inconceivably old, still living, which, if they did not belong to other worlds than ours, date from a time when the world was other than it is now; and the fresh-water plants, equally prehistoric, on which these creatures feed.