The chief agents in the decay of animal and plant organisms are air, rain-water, and atmospheric moisture, as dew, mist, and frost. We can see the destructive effects of air in the case of canned meats or fruits. If the sealing of the can is imperfect, the air penetrates, and carries some germ which corrupts the contents. All the vegetable remains that cover the ground each autumn, do not accumulate year after year, but speedily decay and become soil. The animals and insects, large and small, that die in the fields and woods each year, do not long remain where they fell; carnivorous birds and small animals, especially ants and beetles, devour the soft portions, and a few years of exposure to air and moisture reduce the bony skeletons to powder. As the thing that is, is generally the thing that hath been, and shall be, we may conclude that nearly all the animals and plants of each age perished with their age, and that what remain as fossils are but a small fragment of the whole, and have been preserved in unusually favorable circumstances. Ice, as we have seen, has served as a wonderful preservative, but the instances where animals or plants have been subjected to the ice-process are naturally few. Next to ice, peat mosses have exhibited remarkable preserving qualities, and have kept entire remains, as deer, bison, oxen, buried in them. Heavy beasts becoming mired in the peat, have sunk into it to a great depth, and were hermetically sealed from the oxygen of the air, and from spores called _bacteria_[29] which are the chief agents of decay. Many fossil organisms are preserved in the silt or débris at the mouths of rivers, where the deposit of earthy matter carried down the stream is rapid, and speedily buries up whatever is dropped upon it. On the floors of lakes also, there are often deep deposits of marl, peat, and sand, and these sometimes bury and preserve the bodies of birds or other animals, and the seeds, branches, or entire plants, that have been swept into the water. Caverns, especially those in limestone or chalk rock, have proved famous treasure-houses of fossils. Where, in these caverns, there is a dripping of water charged with lime or chalk, it speedily encloses objects in a safe case which effectually preserves them. Sometimes also these bone caves, ancient homes of wild beasts, have been closed up by silt or earth-falls, and have retained many specimens of large, ancient fauna now extinct. [Illustration: DONE IN STONE.]