But there is no magic in Nature; and when we meet with undoubted phenomena, however strange they may appear, in any department of her realm, we must behave ourselves like matter-of-fact persons and set about accounting for them logically. In a genuine petrifaction there are two ways of doing this, or, in other words, there are two different modes in which Nature may have acted so as to produce the result which we are considering. One of these is that of a simple _substitution_; the particles of one substance being gradually removed, and those of another taking their place. This must have been the process which obtained in many well-known instances: as, for example, when the thin, leathery shell or husk of an echinus is found perfectly rendered, both as to its general outline and minute markings, in solid flint or limestone. We feel quite sure that the original husk has perished; but here is its second self. The other mode is that of actual _transmutation_, wherein no part of the original substance is destroyed or removed, but its physical conditions undergo a total change, by means of either infiltration, or crystallization, or perhaps both of them combined. This process has obtained in such cases as a petrified shark’s tooth, or a fossil trilobite from the Wenlock. The tooth, which was once bone, is now a kind of metallic stone: the trilobite once, as we suppose, shell and cartilage, is now a something between limestone and cast-iron. And this change is very wonderful; but, as was said above, there is no magic in it, no more than in the hardening of an infant’s skull, or the solidifying of the arm and leg-bones by the absorption of phosphate of lime. Besides, the stone and metal now present are still something different from what we ordinarily mean by those terms: the petrified tooth is not like a flint-stone, neither could you cut horse-shoe nails out of the trilobite. In fact, every _real_ process has a character of its own: a something which distinguishes it from any other, despite of general resemblances up to a certain point. Petrifaction, then, being assumed, as an operation which has really taken place after one way or another, it falls next to be considered, what was the original nature of these fossils? I mean the objects which we find thus preserved in our seaside pebbles. What was the department in Creation to which they belonged? Was it _animal_ or _vegetable_ before it thus became altogether _mineral_?