The next question which presents itself is the geography of Northwestern Europe, while the above strange group of animals lived in Southern Britain. It is obvious from the fact of the above animals finding their way here that our island must then have formed part of the Continent. The fluviatile strata of Crayford have been met with at a depth of forty feet below high-water mark near Erith, so that then the whole lower portion of the Thames Valley was higher above the sea than it is now. The land, however, must have stood at a considerably higher level than that, since the soundings in the shallowest part of the Channel reveal a depth of about two hundred feet, and therefore an elevation of land of more than two hundred feet is necessary to allow of the migration of the lion and the other animals. The area now covered by the "silver streak" was then composed of forest-clad undulations, extending from the line of the chalk downs then reaching from Dover to Sangatte, in the Pas de Calais, on the one hand, northward into the fertile pastures now sunk beneath the North Sea, and on the other, to the southwest along the whole length of the Channel. Nor are we able to find evidence of the western sea-margin at this time till the hundred-fathom line is reached, which sweeps far to the west of Ireland, southward close into the shores of the Bay of Biscay, and northward so as to include the Hebrides and the Orkneys, forming a narrow fiord close to the present coast of Norway, that reaches as far as Denmark. The view of De la Bêche and Lyell, that all within this boundary was dry land, only broken by the rivers and the lakes, is most probably true. In this manner alone can we account for the presence of some of the above animals, such as the spotted hyena, in Ireland.
But when, it will be asked, were these things so? The answer is found in the fact of the presence of the living species of higher mammalia along with certain extinct species such as the mammoth, which points to one, and one only, stage in the evolution of animal life—that which is termed Pleistocene or Quaternary by the geologists, and further, to the middle stage of it, when temperate animals abounded and Arctic animals were rare in Southern Britain. The question is unanswerable if asked from the historical and not the geological point of view, because, outside the records in which the intervals between events are written down, we have merely a series of events which occurred in a certain order, without reference to lapse of time. An attempt to ascertain an historical date outside history is obviously idle, and is not furthered by an appeal to the present rate of the retrocession of waterfalls, or by speculations as to ancient changes in climate having been produced by changes in the relation of the earth to the sun. The events with which we are dealing—the conditions of life when the lion first appeared in Britain—are so far removed from the earliest records that we can not form an idea of the interval separating them from our own time. It must, however, have been very great to allow of the