"The contribution of Karl Marx and Friedrich Engels to socialism was a theory that purported to show why the kingdom of equality was not only desirable and feasible but inevitable. To advance this claim, they resorted to methods borrowed from the natural sciences, which had gained immense prestige in the nineteenth century. Marx and Engels formulated a doctrine of “scientific socialism,” which asserted that the ideal of a propertyless, egalitarian society was something that not only should happen but, by virtue of the natural evolution of the economy, had to happen. The Marxist concept of social evolution arose under the influence of the Darwinian theory formulated in 1859 in On the Origin of Species. Darwin’s book depicted the emergence of biological species as due to a process of natural selection that enabled them better to survive in a hostile environment. The process was a dynamic one, evolving species from lower to higher stages according to determinable rules. This theory was quickly adapted by students of human behavior, giving rise to a school of “evolutionary sociology” that depicted history as a progression, “by stages,” from lower to higher forms. So great was Darwin’s influence on Marx that Engels, speaking at his friend’s funeral, said, “Just as Darwin had discovered the law of development of organic nature so did Marx discover the law of human history.”"