James Rachels, answered from the texts and cited to the page.
Traditional morality depends on the doctrine of human dignity: the idea that human life has a special, unique value, while non-human life has relatively little value.12 The doctrine has historically rested on two supports — that man is made in the image of God, and that man is a uniquely rational being. Darwinism does not formally entail that human dignity is false (the is/ought stricture forbids the inference from facts about evolution to conclusions about value); but Darwinism undermines the doctrine by removing the supports it has traditionally rested on.3
After Darwin, man is no longer a "great work" worthy of the interposition of a deity; he is a product of the same evolutionary forces that shaped the rest of the animal kingdom. The image-of-God conception fails. The uniquely-rational-being conception fails too: the empirical evidence on continuity (Chapter 4) shows that the differences between humans and other animals in the capacities traditionally invoked to ground human dignity are differences of degree, not of kind.45
The doctrine collapses. To replace it, Created from Animals offers moral individualism: the position that the bare fact that one is human entitles one to no special consideration. How an individual should be treated depends on his or her own particular characteristics, rather than on whether he or she is a member of some preferred group — even the "group" of human beings.
The Argument from Marginal Cases establishes the position negatively: cognitively impaired humans don't possess the capacities traditionally invoked to ground human dignity, yet we accord them moral consideration; some non-human animals possess these capacities to a higher degree than some humans, yet we don't accord them comparable consideration; the asymmetry is unjustifiable on any criterion except species-membership, which moral individualism rejects.
The argument may be summarized briefly: 1. Traditional morality depends on the idea that human beings are in a special moral category: from a moral point of view, human life has a special, unique value, while non-human life has relatively little value. Thus the purpose of morality is conceived to be, primarily, the protection of human beings and their rights and interests.Created from Animals - The Moral Implications of Darwinism, pp. 4–5
Can it really be true that Darwinism, which overturns all our former ideas about man and nature, has no unsettling consequences? Traditional morality is based, in part, on the idea that human life has a special value and worth.Created from Animals - The Moral Implications of Darwinism, p. 4
The claim implicit in traditional morality is that humans are morally special because they are made in the image of God, or because they are uniquely rational beings. We are now in a position to explain how Darwinism might undermine traditional morality. The claim that Darwinism undermines traditional morality is not the claim that it entails that the doctrine of human dignity is false.Created from Animals - The Moral Implications of Darwinism, pp. 97–99
RECAPITULA T I O N The main thesis o f this book i s that Darwinism leads inevitably to the abandonment of the idea of human dignity and the substitution of a different sort of ethic. In Chapter 2, I outlined the strategy that I would use in arguing for this claim.Created from Animals - The Moral Implications of Darwinism, pp. 171–173
Nevertheless, ifhe was right about even part of this, it would follow that, often, when we object to treating a human in a certain way, we would have similar grounds for objecting to the similar treatment of a non-human animal.Created from Animals - The Moral Implications of Darwinism, p. 175
Moral individualism is Rachels's most distinctive positive theoretical position and the most-debated. It has fellow-travelers — Peter Singer's preference utilitarianism arrives at similar practical conclusions; Tom Regan's rights-based animal ethics arrives at adjacent conclusions through different theoretical apparatus; the Argument from Marginal Cases is shared across the animal-ethics literature. It has serious opponents — Carl Cohen's defense of speciesism (the view that species-membership IS morally relevant in itself); the religious-dignity defenders (the Catholic and Protestant traditions); and the virtue-ethical / relationship-based critiques that argue moral consideration tracks relational ties rather than individual characteristics. The contemporary debate centers on whether "moral individualism" is a substantive normative position distinguishable from preference utilitarianism, or whether it is a methodological commitment to characteristic-based reasoning that leaves the substantive normative question open. The position defends moral individualism as the substantive replacement for human dignity that Darwinism makes necessary, and engages Cohen, the religious-dignity tradition, and the virtue-ethical critique by name.