Introgressive hybridization, the feeding back of alien genes from an invading species into a resident population, has not been widely studied in animals. Conclusions favouring its occurrence are mostly founded on comparative studies of external morphology: since parallel selection of similar gene allels would provide the same end result, such conclusions are open to question. White's arguments against introgression as the cause of overlapping phenotypes in Austroicetes grasshoppers, on the other hand, carry conviction because they are based on comparative internal morphology – on cytological study: the two supposedly interbreeding species were found to differ in several chromosomal diagnostics, to the complete absence of intermediates.