A few years ago I set out to canvass the literature on Charles Darwin. I thought it would be a manageable task, but I soon realised what a naive idea this was. I do not know how many books have been written about him, but there seem to be thousands, and each year more appear.[1] Why are there so many? Part of the answer is, of course, that he was a tremendously important figure in the history of human thought. But as I read the books - or, at least, as many of them as I could - it gradually dawned on me that all this attention is also due to Darwin's personal qualities. He was an immensely likeable man, modest and humane, with a personality that continues to draw people to him even today.

Reflecting on his father's character, Darwin's son Francis wrote that The two subjects which moved my father perhaps more strongly than any others were cruelty to animals and slavery. His detestation of both was intense, and his indignation was overpowering in case of any levity or want of feeling on these matters.'[2] Darwin's strong feelings about slavery are expressed in many of his writings, most notably in the Journal of Researches, in which he recorded his adventures on the Beagle voyage. His comments there are among the most moving in abolitionist literature. But it was his feelings about animals that impressed his contemporaries most vividly. Numerous anecdotes show him remonstrating with cab-drivers who whipped their horses too smartly, solicitously caring for his own animals and forbidding the discussion of vivisection in his home.[3] At the height of his fame he wrote an article for a popular magazine condemning the infamous leg-hold trap in terms that would not seem out of place in an animal-rights magazine today.

For the most part, however, Darwin avoided moralising in his scientific books. Earlier students of nature had viewed the natural order as a kind of moral laboratory in which God's design was everywhere evident, and so they found all manner of moral lessons there. Darwin believed it is a mistake to think about nature in this way. Nature is 'red in tooth and claw'. Rather than embodying some great moral design, nature operates by eliminating the unfit in ways that are often cruel and that do not conform to any human sense of right.

Nevertheless, Darwin did think that something can be learned about morality from the scientific study of human origins. The third chapter of his great work The Descent of Man is an extended essay on morality, 'approached', as he put it, 'from the side of natural history'.[4] In that chapter Darwin discusses, among other things, the nature of morality, its biological basis, the extent of our moral duties, and the prospects for moral progress. It is the work of a moral visionary as well as a man of science.

Darwin's remarks about moral progress are especially striking. We are moral beings because nature has provided us with 'social instincts' that cause us to care about others. (The social instincts are, of course, produced by natural selection, as are almost all our traits.) At first, though, the reach of the social instincts does not extend very far - we care only about our near kin and those whom we can expect to help us in return. Moral progress occurs over time as the social instincts are extended ever more widely, and we come to care about the welfare of more and more of our fellow beings. The highest level of morality is reached when the rights of all creatures, regardless of race, intelligence, or even species, are respected equally:

[T]he social instincts which no doubt were acquired by man, as by the lower animals, for the good of the community, will from the first have given to him some wish to aid his fellows, and some feeling of sympathy. Such impulses will have served him at a very early period as a rude rule of right and wrong. But as man gradually advanced in intellectual power and was enabled to trace the more remote consequences of his actions; as he acquired sufficient knowledge to reject baneful customs and superstitions; as he regarded more and more not only the welfare but the happiness of his fellow-men; as from habit, following on beneficial experience, instruction, and example, his sympathies became more tender and widely diffused, so as to extend to the men of all races, to the imbecile, the maimed, and other useless members of society, and finally to the lower animals, - so would the standard of his morality rise higher and higher.[5]

The virtue of sympathy for the lower animals is 'one of the noblest with which man is endowed'.[6] It comes last in the progression because it requires the greatest advancement in thought and reflection.

What are we to make of this? One possibility is to say that Darwin's moral attitudes were separate from, and independent of, his strictly scientific achievement. In opposing slavery, he was properly sympathetic to one of the great moral movements of his times. In opposing cruelty to animals, he showed himself to be kind-hearted, as we all should be. But no more should be made of it than that. On this way of thinking, the moral views expressed in The Descent of Man are just extra baggage, having no more to do with the theory of natural selection than Einstein's reflections on war and peace had to do with special relativity. Like other nineteenth-century writers, Darwin could not resist presenting his thoughts about ethics alongside his scientific work. But we, at least, should keep the two things separate.

There is, however, another way of thinking about Darwin's life and work. Perhaps his scientific work and his moral views were connected, as he apparently believed they were, in a significant way. If so, then we may have reason to view them as one piece, and it may not be so easy to embrace the one without the other. Asa Gray, the Harvard botanist who was Darwin's leading defender in America, took this view. Speaking before the theological faculty at Yale in 1880, Gray declared that

We are sharers not only of animal but of vegetable life, sharers with the higher brute animals in common instincts and feelings and affections. It seems to me that there is a sort of meanness in the wish to ignore the tie. I fancy that human beings may be more humane when they realize that, as their dependent associates live a life in which man has a share, so they have rights which man is bound to respect.[7]

Asa Gray had identified the essential point. Darwin had shown that all life is related: we are kin to the apes. If this is true, then if we have rights, would it not follow that they have rights as well?

Let me try to explain this point in a little more detail. A fundamental moral principle, which was first formulated by Aristotle, is that like cases should be treated alike. I take this to mean that individuals are to be treated in the same way unless there is a relevant difference between them. Thus if you want to treat one person one way, and another person a different way, you must be able to point to some difference between them that justifies treating them differently. Where there are no relevant differences, they must be treated alike.

Aristotle's principle applies to our treatment of nonhumans as well as to our treatment of humans. Before Darwin, however, it was generally believed that the differences between humans and nonhumans are so great that we are almost always justified in treating humans differently. Humans were thought to be set apart from the rest of creation. They were said to be uniquely rational beings, made in God's image, with immortal souls, and so they were different in kind from mere animals. It is this picture of humankind that Darwin destroyed. In its place he substituted a picture of humans as sharing a common heritage, and common characteristics, with other animals.

If we take the Darwinian picture seriously, it follows that we must revise our view about how animals may be treated. It does not follow that we must treat all animals as the equals of humans, for there may still be differences between humans and some animals that justify a difference in moral status. It would make no sense, for example, to argue that clams should be given the right to live freely, because they lack the capacity for free action. Or perhaps the members of some species, such as insects, lack even the capacity for feeling pain, so that it would be meaningless to object to 'torturing' them. Other examples of this type may come easily to mind.

Nevertheless, when we turn to the 'higher' animals, such as the great apes, it is the similarities and not the differences between them and us that are so striking. These similarities are so widespread and so profound that often there will be no relevant differences that could justify a difference in treatment. Darwin argued that such animals are intelligent and sociable and that they even possess a rudimentary moral sense. In addition, he said, they experience anxiety, grief, dejection, despair, joy, devotion, ill-temper, patience, and a host of other 'human' feelings. Ethological studies since Darwin's day have confirmed this picture of them. The moral consequence is that if they have such capacities, then there is no rational basis for denying basic moral rights to them, at least if we wish to continue claiming those rights for ourselves. Chief among those rights are the right to life, the right to live freely and the right not to be caused unnecessary suffering.

It would be easy to overstate this conclusion and to misrepresent its basis. The conclusion is not that the great apes should be granted all the rights of humans or that there are no important differences between them and us. There may still be some human rights that have no analogues for the apes. In an enlightened society, for example, humans are granted the right to higher education. Because reading is essential for acquiring such an education, and not even the most intelligent non-humans can read, it makes no sense to insist that they be given this right. But the right to live freely and the other basic rights mentioned above do not depend on the ability to read or on any other comparable intellectual achievement, and so such abilities are not relevant to eligibility for those rights. Aristotle's principle requires equal treatment where, but only where, there are no relevant differences.

Partisans of the animal rights movement sometimes represent such conclusions as based on the genetic similarity between humans and other apes. But the importance of this fact is easily misunderstood. Shared DNA is further proof of our kinship with other animals, it confirms the Darwinian picture, but it is not the bare fact that we share genetic material with the chimps that forces the moral conclusion. What forces the moral conclusion is that the chimps, and other great apes, are intelligent and have social and emotional lives similar to our own. Genes are important, to them and to us, only because they make those lives possible.

Before Darwin, the essential moral equality of the great apes — a category that, of course, includes us as well as the chimps, gorillas and orang-utans - would have been a surprising claim, difficult to defend. But after Darwin, it is no more than we should expect, if we think carefully about what he taught us. Every educated person has now learned Darwin's lesson about the origins of human life and its connection with nonhuman life. What remains is that we take its moral implications equally seriously. Darwin himself was optimistic:

Looking to future generations, there is no cause to fear that the social instincts will grow weaker, and we may expect that virtuous habits will grow stronger, becoming perhaps fixed by inheritance. In this case the struggle between our higher and lower impulses will be less severe, and virtue will be triumphant.[8]