My object will be, first, to show by what connections the history of the fossil bones of land animals is linked to the theory of the earth and why they have a particular importance in this respect.

The appearance of the bones of quadrupeds, especially those of complete bodies in the strata, tells us either that the layer itself which carries them was in earlier times dry land or that dry land was at least formed in the immediate area.

It has been long considered possible to explain the more ancient revolutions on... the Earth surface by means of these still existing causes; in the same manner as it is found easy to explain past events in political history, by an acquaintance with the passions and intrigues of the present day. But we shall presently see that unfortunately this is not the case in physical history:-the thread of operation is here broken, the march of nature is changed, and none of the agents that she now employs were sufficient for the production of her ancient works.

Secondly, the nature of the revolutions which have altered the surface of the earth must have had a more decisive effect on the terrestrial quadrupeds than on the marine animals.

At the sight of a single bone, of a single piece of bone, I recognize and reconstruct the portion of the whole from which it would have been taken. The whole being to which this fragment belonged appears in my mind's eye.

Hence the same instant which killed the animals froze the country where they lived. This event was sudden, instantaneous, without any gradual development.

All organs of an animal form a single system, the parts of which hang together, and act and re-act upon one another; and no modifications can appear in one part without bringing about corresponding modifications in all the rest.