The centre of population has passed northward since the establishment of Christianity in the Roman Empire.

It has since passed westward, in consequence of the development of manufacturing industry.
We may now examine somewhat more minutely the character of the resistances which thus, for a thousand years, kept the population of Europe stationary.

The surface of the Continent was for the most part covered with pathless forests; here and there it was dotted with monasteries and towns.

In the lowlands and along the river-courses were fens, sometimes hundreds of miles in extent, exhaling their pestiferous miasms, and spreading agues far and wide.

In Paris and London, the houses were of wood daubed with clay, and thatched with straw or reeds.
They had no windows, and, until the invention of the saw-mill, very few had wooden floors.