The quotation from Kant is from a passage in his Critique of Practical Reason, which is immediately followed by a passage in which Kant writes about the sense of duty or "ought" as showing us "man as belonging to two worlds"--the empirical (phenomenal) world of natural causes and the transcendental (noumenal) world of moral freedom. By contrast, Darwin indicates that his explanation of morality will be "exclusively from the side of natural history." A careful reader might see here a fundamental difference between the Kantian approach that sees morality as belonging to a transcendent world beyond the natural world and the Darwinian approach that sees morality as belonging completely to "natural history."
In fact, some of Darwin's first readers saw this, and many of them were deeply disturbed by it. For example, Frances Cobbe wrote a review of the book--"Darwinism in Morals"--for the Theological Review, in which she warned that Darwin's rejection of the Kantian view of morality as transcending natural human experience would destroy all morality.
Cobbe saw a fundamental conflict between two views of morality. "Independent or Intuitive morality has, of course, always taught that there is a supreme and necessary moral law common to all free agents in the universe, and known to man by means of a transcendental reason or divine voice of conscience. Dependent or Utilitarian Morality has equally steadily rejected the idea of a law other than the law of utility" (5-6). Darwin clearly takes the second position. She observed "that the Kantian doctrine of Pure Reason, giving us transcendental knowledge of necessary truths, is not entertained by the school of thinkers to which he belongs; and that as for the notion of all the old teachers of the world, the voice of Conscience is the voice of God--the doctrine of Job and Zoroaster, Menu and Pythagoras, Plato and Antonius, Chrysostom and Gregory, Fenelon and Jeremy Taylor,--it can have no place in their science. As Comte would say, we have passed the theologic stage, and must not think of running to a First Cause to explain phenomena. After all (they seem to say), cannot we easily suggest how man might acquire a conscience from causes at work around him?" (7)
Darwin maintained that although the moral sense was unique to human beings, it would be possible for evolutionary history to produce another species of animal with a different kind of moral sense. "Any animal whatever, endowed with well-marked social instincts, the parental and filial affections being here included, would inevitably acquire a moral sense or conscience, as soon as its intellectual powers had become as well, or nearly as well developed, as in man." But the content of the moral sense in such an animal would depend upon the desires and needs of the animal. Darwin explained:
"If, for instance, to take an extreme case, men were reared under precisely the same conditions as hive-bees, there can hardly be a doubt that our unmarried females would, like the worker-bees, think it a sacred duty to kill their brothers, and mothers would strive to kill their fertile daughters; and no one would think of interfering. Nevertheless, the bee, or any other social animal, would gain in our supposed case, as it appears to me, some feeling of right and wrong, or a conscience. For each individual would have an inward sense of possessing certain stronger or more enduring instincts, and others less strong or enduring; so that there would often be a struggle as to which impulse should be followed; and satisfaction, dissatisfaction, or even misery would be felt, as past impressions were compared during their incessant passage through the mind. In this case an inward monitor would tell the animal that it would have been better to have followed the one impulse rather than the other. The one course ought to have been followed, and the other ought not; the one would have been right and the other wrong" (122-23).
It seems, than, that the moral sense or conscience--the sense of moral "ought"--is a "feeling of right or wrong" that varies according to the instinctive desires of the species. So far, the human moral sense is the only moral sense, because human beings are the only animals with the evolved capacities for moral judgment. But if any other species of animal were to evolve such moral capacities, their moral sense would differ from the human moral sense depending upon the differences in their instinctive desires.
Cobbe was appalled by this--by the claim that if bees had a moral sense, it would prescribe a sacred duty for sisters to murder their brothers. She saw this as "affirming that, not only has our moral sense come to us from a source commanding no special respect, but that it answers to no external or durable, not to say universal or eternal, reality, and is merely tentative and provisional, the provincial prejudice, as we may describe it, of this little world and its temporary inhabitants, which would be looked on with a smile of derision by better-informed people now residing on Mars, or hereafter to be developed on earth, and who in their turn may be considered as walking in a vain shadow by other races" (10). She warned: "Our moral sense, however acquired, does not, it is asserted, correspond to anything real outside of itself, to any law which must be the same for all Intelligences, mundane or supernal" (28).
Against what she perceived as Darwin's moral nihilism, Cobbe asserted that ethics was a normative science just like geometry in that both ethics and geometry were based on axiomatic principles that all intelligent beings could recognize as necessary truths. "Love your neighbor" is such a necessary truth of morality, and therefore any intelligent being should eventually understand that moral duty dictates universal love, which would be as true for bees as for humans (29).
Other readers besides Cobbe--including George Jackson Mivart and even Alfred Russel Wallace (the co-discoverer of the idea of natural selection)--warned that Darwin's evolutionary account of morality denied the eternal truth of morality as rooted in the transcendent cosmic order of God, Reason, or Nature. The same warning is heard today from proponents of "intelligent design theory" (like John West and Richard Weikart), who insist that morality cannot be sustained if it is not grounded in some transcendent world of moral truth beyond the empirical world of natural causes.
The reasoning for this worry that evolutionary ethics promotes moral nihilism--or at least moral skepticism--has been elaborated by philosopher Richard Joyce in two books: The Myth of Morality (2001) and The Evolution of Morality (2006). Joyce agrees with Cobbe (and Kant) that by definition moral judgments presuppose belief in a transcendent world of moral facts beyond the empirical world of natural facts. But since he denies the truth of that belief, because there really are no such moral facts, he concludes that we cannot know that morality is true, and therefore we cannot know that moral rightness and wrongness really exist. Moreover, he argues that evolutionary ethics necessarily leads us to this conclusion that morality is fictional, because an evolutionary account of morality explains it as arising from the natural facts of human desires and capacities without any reference to any distinctively moral facts.
Joyce believes that Darwin's evolutionary ethics explains the ultimate causes of ethics in a way that confirms David Hume's insight that the human moral sense depends on the human tendency to project emotions onto the world. Hume spoke of the mind's "great propensity to spread itself on objects," because "taste" (as opposed to reason) "has a productive faculty, and gilding and staining all natural objects with the colours, borrowed from internal sentiment, raises in a manner a new creation." So it is that "vice and virtue may be compared to sounds, colours, heat and cold, which, according to modern philosophy, are not qualities in objects but perceptions in the mind." And yet, our human moral principles can be enduring, if not universal, insofar as our moral emotions reflect the "universal constitution of human nature." For Hume, the explanation of the origins of human morality ended with some final appeal to human nature as ultimate cause, because he could not go any further to explain the original causes of human nature itself. But here is where Darwin could adopt Hume's account of the natural moral sense, but then go further in developing an evolutionary explanation for how human nature as endowed with a moral sense could have arisen.
But while Joyce accepts this Humean/Darwinian account of morality as an evolved propensity of human nature to project human moral emotions onto the world, he cannot believe that this is a full explanation of morality, because he agrees with Kant that moral concepts necessarily presuppose belief in a transcendent world of moral facts. Like many moral philosophers--from Plato to Kant--Joyce embraces a transcendental view of morality in contrast to the empirical view of morality taken by those like Hume and Darwin. In other words, Joyce agrees with Reme Brague that morality requires a moral cosmology, so that morality can be understood as having an eternal truth as corresponding to some cosmic order of God, Reason, or Nature. But since Joyce denies the reality of that moral cosmos, he must conclude that morality cannot be known to be true.
In this way, Joyce is like Nietzsche, who accepted the Platonic/Kantian teaching that morality must be grounded in some transcendent reality of the Idea of the Good, Pure Reason, or Divine Law, but who denied the truth of that belief in transcendent moral order, and therefore worried that moral nihilism was inevitable. Nietzsche's response to this dilemma was to try to create a new myth with a new religion of Dionysus that would support a new morality of nobility as commanded by new philosophers shaping a new human future.
Joyce's response to his similar dilemma is unclear. On the one hand, he insists that morality is impossible without the "moral clout" that comes from believing in the eternal truth of moral facts, which provides the strength of will to overcome the weakness of will that plagues us when we rely on merely prudential reasoning about what is desirable for us. On the other hand, he also insists that human beings have evolved motivations for good conduct even when they don't believe in God or eternal moral duty (2006: 224).
Joyce admits that a strong sense of transcendent moral duty can often support an authoritarian morality that does great damage. After all, human history gives us many examples of atrocities committed by people who thought they were doing their moral duty.
As I have indicated in previous posts, many of the Nazi philosophers were neo-Kantians who believed in "eternal values" and in Nazism as fulfilling that eternal moral order. Moreover, Claudia Koonz's book The Nazi Conscience (2003) shows how the Nazi regime was organized around a strict communitarian morality of sacrificing selfish interests for the good of the community. Also, Jonathan Glover's moral history of the 20th century shows how the greatest atrocities were committed by those moved by a fanatical utopian Belief in the goodness of their cause.
Joyce's answer to this problem is to say that this only shows the danger in wrong moral beliefs! But this makes us wonder how the purity of moral judgment as separated from prudential judgment can be preserved if we have to judge the rightness or wrongness of moral beliefs. According to Joyce, morality "functions to bolster self-control. It imbues certain desirable actions with a 'must-be-doneness,' which raises the likelihood of their being performed. . . . It goes without saying that if this 'must-be-doneness' were attached to the wrong actions--undesirable ones--then it would be disastrous in practical terms" (2001: 181).
But now it seems that Joyce is subordinating his categorical imperatives of moral duty to the hypothetical imperatives of prudential calculation, because now it seems that we don't know whether this "must-be-doneness" really must be done until we have judged that the outcome will be desirable for us! If so, then Joyce ends up agreeing with me that "the good is the desirable."
Joyce's reasoning would be clearer to me if he had compared his position with Edward Westermarck's. Westermarck's elaboration of Darwinian evolutionary morality is remarkably similar to Joyce's--particularly, in the claim that morality arises from evolved moral emotions and therefore that moral concepts have no objectivity insofar as they have no real existence apart from the human mind. But while Westermarck develops his Darwinian ethics as a total rejection of Kant's transcendentalist "two worlds" view, Joyce agrees with Kant that moral concepts by definition must refer to some transcendental realm of eternal cosmic order.
Like Westermarck, I am on the side of Darwin against Cobbe. I agree that believing in a moral law grounded in divine will or transcendent reason might strengthen the moral motivation of people with such a belief. But it seems clear to me that people can still have a strong motivation for moral conduct when they believe that morality has no other ground than the evolved human nature of our moral emotions.
By contrast, Joyce can't decide whether he's on Darwin's side or Cobbe's side. He seems to agree with Darwin that human morality can be fully explained "exclusively from the side of natural history." But then he also seems to agree with Cobbe that human morality cannot be fully explained without some belief in "the Kantian doctrine of a Pure Reason, giving us transcendental knowledge of necessary truths," or in the idea that "the voice of Conscience is the voice of God."
A few of the many posts on related themes can be found here, here, here, here, here, here, here, here, and here.