In this book, Peter Singer shows how the Left, assuming that all inequalities are due to discrimination, prejudice, oppression, or social conditioning, misunderstood Darwinism, and that it is time for the Left to take seriously the fact that we are evolved animals and that the evidence of our inheritance exists not only in our anatomy and DNA, but also in our behavior. A pre-Darwinian Left would attribute the existence of fraudsters to poverty, or lack of education, the inheritance of "reactive capitalist ways of thinking". The political lesson of 20th century Darwinian thought is completely different from that of 19th century social Darwinism, because now understanding the conditions of mutual cooperation, as well as its advantages, a new Darwinian Left can fight to avoid the economic conditions that create outcasts. Starting from the contemporary data of evolutionary theory, the author examines the problems of cooperation at a social level and the puzzle of altruism, which not infrequently comes into conflict with one's individual interest and well-being.