Differentiation
N.H. Barton, in:, Encyclopedia of Biodiversity, 2nd ed., Elsevier, 2013, pp. 508–515.

Electrophoretic analysis of a chromosomal hybrid zone in the grasshopper Podisma pedestris
B. Halliday, N.H. Barton, G. Hewitt, Biological Journal of the Linnean Society 19 (1983) 51–62.