But we shall follow the changes more readily, by supposing a new glacial period slowly to come on, and then pass away, as formerly occurred.

As the cold came on, and as each more southern zone became fitted for the inhabitants of the north, these would take the places of the former inhabitants of the temperate regions.

The latter, at the same time would travel further and further southward, unless they were stopped by barriers, in which case they would perish.

The mountains would become covered with snow and ice, and their former Alpine inhabitants would descend to the plains.

By the time that the cold had reached its maximum, we should have an arctic fauna and flora, covering the central parts of Europe, as far south as the Alps and Pyrenees, and even stretching into Spain.