marxism

To tack onto what @Maoo@hexbear.net said, it is this form of commodification of labour that will eventually feed into the Leninist theory of imperialism. Imperialism isn't when you take land, as land has no inherent value to the capitalist, imperialism is when you create a zone of enforced and tiered commodified labor based off of the geographic location or citizenship. Essentially, what made the Romans rich wasn't land, it was slaves. But that is more advanced theory than what is being spoken of here. Perhaps easier to grasp, but what is important here is the math and the axiomatic assumptions underlying the argument.

What is being said here is that migration is caused by the very economic circumstances that create population surplus.

Basically the math is this. If we assume labor is a commodity (you have to sell your labor to live), and that the area which you live has only a fixed amount 'work' to be sold, or an amount of 'work' that does not increase commensurate to the population growth, then by definition there will not be enough 'work' to support the relative population surplus of laborers. These laborers that are left without work, will have to then migrate to areas where 'work' is available to be sold. Capitalism then, presupposes the ability of people to migrate to where work is in order to function.

It is from there that we can then discuss, if that is a presumption of capitalism, why then are migrant laborers (an assumed part of the economic system) always given the short end of the stick, rather than considered an essential part of the capitalist system, an investigation of contradictions which is what then leads into ideas of Leninist imperialism and tiered exploitation models.