1. The Species: Then and Now
2. BARTON, N. H., AND G. M. HEWITT. 1983. Hybridzones as barriers to gene flow, p. 341-359. In G.S. Oxford and D. Rollinson [eds.], Protein poly-morphism: adaptive and taxonomic significance.Blackwells, Oxford.BartonHybrid zones as barriers to gene flow341Protein polymorphism: adaptive and taxonomic significance1983