From what I recall reading, there was a grow in population during XIX century that to this day is still hard to explain by historians. A lot of that surplus population couldn't live off the land, since it was already at capacity in most of the Western Europe, so they moved to cities, hoping to find a living there. This resulted in "infernal cities" of XIX century, as described by Dickers, Zola etc - people dying of starvation of right on the streets of London or Paris, hordes of underage prostitutes roaming the streets etc.