In working out the geographical conditions for any particular epoch of the earth’s history, it is possible to go much farther than merely gaining an approximate estimate of the distribution of land and sea; many other important facts may be gathered from a minute examination of the rocks in combination with a genetic study of topographical forms. By this physiographical method, as it is called, the history of several of the great mountain-ranges has been elaborated in great detail. It is quite practicable to give a geological date for the initial upheaval and to determine whether one or many such series of movements have been involved in bringing about the present state of things. Similarly, the history of plains and plateaus, hills and valleys, lake and river systems, may be ascertained, and for the earth’s later ages, at least, a great deal may be learned regarding the successive forms of the land-surfaces in the various continents. It would be very desirable to explain the methods by which these results are reached, but this could hardly be done without writing a treatise on physiography, for which there is no room in this chapter. We must be permitted to make use of the results of that science without being called upon to prove their accuracy. No factor has a more profound effect in determining the character and distribution of living things than climate, of which the most important elements, for our purpose, are temperature and moisture. One of the most surprising results of geological study is the clear proof that almost all parts of the earth have been subjected to great vicissitudes of climate, and a brief account of the evidence which has led to this unlooked for result will not be out of place here.