No one will doubt, that within the last ten years tens of thousands of the common white butterfly have disported themselves in England, yet a man might easily starve if he were allowed no food till he had found some of their fossil remains. The dodo has not long been extinct, but nevertheless fossil dodos are extremely rare. It may be thought that the date of extinction has little to do with the matter, and that each relic when enshrined in the rock, may claim to be by a sort of indelible character ‘once a fossil and always a fossil.’ This, however, is in reality far from its true condition. Let a creature’s remains escape being devoured, or burnt, or trampled to pieces, or being dissolved by the rain, or crumbled into dust by rolling waves and mud and stones gathered upon them, their perils are not yet over. Even in the grasp of the hard rock, the fossil may be horribly distorted by pressure, split asunder by cleavage, boiled and baked and crystallized, till none of its features remain what they were, or till the very fact of its presence becomes only the question of a dim surmise. The rude jolt of an earthquake, that splits asunder a mountain, may sometimes be tender over a butterfly’s wing; but there are chemical agencies which work without any compassion for what is fine and delicate, and by these we find great thicknesses of rock apparently stripped of their fossils. Where the whole stratum consists of remains of once living organisms, as in seams of coal, it has been shown that we have no reason to suppose that any complete or adequate memorials are left us of the whole vegetation of any particular period or any particular area; since Dr. Lindley has found, by actual experiment, that different vegetables have very different powers of resisting decay, and that pines and ferns and lycopodia will be well preserved after long immersion in water, while the same treatment causes the disappearance of grasses and sedges, of the oak-tree and the ash[61]. Even those rocks which preserve fossils most carefully may themselves be crumbled to pieces, fossils and all, by the process of denudation.