That there is a continuous link from humans to gorillas, with the intermediate species merely long dead, is beyond the understanding of speciesists. Tie the label Homo sapiens even to a tiny piece of insensible embryonic tissue, and its life suddenly leaps to infinite, incomputable value.... Self-styled pro-lifers, and others that indulge in footling debates about exactly when in its development a foetus becomes human, exhibit the same discontinuous mentality. Human, to the discontinuous mind, is an absolutist concept. There can be no half measures. And from this flows much evil.
Richard Dawkins