According to the right-wing view, Darwin proved that mankind is subject to the rule of the survival of the fittest and from that it was deduced that a perpetual increase in the wealth and power of the top beneficiaries of capitalism cannot be avoided because it follows an ineluctable law of nature. Singer claims that Darwin's work can be interpreted in a different way. Left-wing Darwinism accepts the theory of evolution, of course, but also allows that men are somewhat more malleable than the other animals. Singer argues that the limits of human malleability can be observed by observing different social groups. He notes, thirdly, that societies vary hardly at all in respect to the presence of hierarchy.
Human nature is capable of cooperation as well as competition.
Human nature is also capable of altruism, as is shown by the British willingness to donate blood freely and anonymously for the benefit of complete strangers. I do not know whether Singer's interpretation of Darwinism would have been recognized by Darwin himself. I suspect it might be rejected by philosophical critics of Darwinism such as Mary Midgley and David Stove. Most of the commentators in this book are more decorous than that. Each volume is a collection the aim of which is to praise someone's life work.