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“Is the term for the human species clearly defined? There is no authoritative definition of 'species'. The boundaries between biological populations are, by definition, fluid. If this wasn't the case, evolution could never have taken place – the principle is change, not being static. In particular, the often quoted criterion, that two different species cannot produce 'fertile offspring' is not consistent with reality. Primates produce fertile offspring, often not only between different species (white-handed gibbon and capped gibbon, hamadryas baboons and olive baboons) but also between different defined genera (such as the hanuman langur [Semnopithecus] and the Nilgiri langur [Presbytis]) and, this happens in the wild. The boundary between the 'species' human and other primates is therefore arbitrary from a biological point of view. There is no 'special characteristic feature' that could justify such a boundary. […]” — Martin Balluch