Nor can it be discovered to vary directly in proportion to the combined amount and distribution of wealth, for in sixteenth-century England while the number of the people was increasing wealth was being concentrated in fewer hands almost as fast as it was being created.

It is obvious that sanitation and transportation have a good deal to do with the population of certain areas.

The largest cities of our own times could not have existed in the Middle Ages, for they could not have been provisioned, nor have been kept endurably healthy without elaborate aqueducts and drains.
Other more obscure factors enter in to complicate the problems of population.

Some nations, like Spain in the sixteenth and Ireland in the nineteenth century, have lost immensely through emigration.

The cause of this was doubtless not that the nation in question was growing absolutely poorer, but that the increase of wealth or in accessibility to richer lands made it relatively poorer.