Arrange these contributions to the study of migration, earliest first.

1. Ravenstein sets out his laws of migration
2. Piore publishes Birds of Passage
3. Stark and Bloom set out the new economics of labour migration
4. Portes and Zhou describe segmented assimilation
5. Spain becomes a country of net immigration
6. Global forced displacement passes one hundred million on UNHCR figures

The century-long gap after Ravenstein is not an accident of scholarship. Migration theory was thin for as long as the phenomenon was treated as a simple response to wage differences; the burst of work from the late 1970s follows the discovery that flows persisted when the differentials that supposedly caused them narrowed.