The healthiest way the human race could proceed from a biological and genetic standpoint is to mix ethnicities - mix so-called 'races'. 'Hybrid vigour' produced offspring that are biologically at least as healthy if not healthier than their parents. Breeding from a small or inbred gene pool weakens us. Pure bred dogs suffer hugely increased deformities and illnesses. Interbreeding cousins over generations - well, see 'Deliverance.' Humans are not 'pure' although we have recognisable ethnic differences. It is healthiest for us to mix.