The establishment of the order of succession of the strata, and the correlation of strata of different areas merely pave the way for the geologist. To write the history of the earth during various geological ages, he has to ascertain the physical and climatic conditions which prevailed during the successive geological periods, and to study the various problems connected with the life of each period. In the present chapter an attempt will be made to illustrate the methods which have been pursued in order to write to the fullest degree which is compatible with our present knowledge, the earth-history of various ages of the past. In making this attempt, the physical and climatic conditions may be first considered, and their consideration followed by that of the changes in the faunas, though it will frequently be necessary to refer to one set of conditions as illustrative of the other. It will be assumed here that the great principle of geology, that the modern changes of the earth and its inhabitants are illustrative of past changes, is rigidly true. Reference will be made to this principle in a later chapter, but it is sufficient to state here that the study of the sediments which have been deposited from the commencement of Lower Palæozoic times to the times in which we now live bear the marks of having been formed under physical conditions, which, in the main, are similar in kind to those which prevail upon some part of the surface of the lithosphere at the present day. One of the most important inferences of the stratigrapher relates to the existence of marine or terrestrial conditions over an area at any particular time, and we may, in the first place, consider the evidence which supplies us with a clue to this subject.