What is lee theory about migration?

lee's theory of migration is based on three elements 1. Place of origin 2. Place of destination 3. Intervening obstacles . There are some positive (pull) and some negative (push) factors. These factors vary from person to person. Push factors at the place of origin compel a migrant to emigrate, and pull factors at the place of destination at attract to immirants. Some time pull factors while sometime push factors are resposible for migration .