The idea of social evolution is admirably expressed in the fine phrase of Leibnitz, "The present is the child of the past, but it is the parent of the future."[60] The great seventeenth-century philosopher was not the first to postulate and apply to society that doctrine of flux, of continuity and unity, which we call evolution. In all ages of which record has been preserved to us, it has been sporadically, and more or less vaguely, expressed. Even savages seem to have dimly perceived it. The saying of the Bechuana chief, recorded by the missionary, Casalis, was probably, judging by its epigrammatic character, a proverb of his people. "One event is always the son of another," he said--a saying strikingly like that of Leibnitz. Since the work of Lyell, Darwin, Wallace, Spencer, Huxley, Youmans, and their numerous followers--a brilliant school embracing the foremost historians and sociologists of Europe and America--the idea of evolution as a universal law has made rapid and certain progress. Everything changes; nothing is immutable or eternal. Whatever is, whether in geology, astronomy, biology, or sociology, is the result of numberless, inevitable, related changes. Only the law of change is changeless. The present is a phase only of a great transition process from what was, through what is, to what will be. The Marx-Engels theory is an exploration of the laws governing this process of evolution in the domain of human relations: an attempt to provide a key to the hitherto mysterious succession of changes in the political, juridical, and social relations and institutions of mankind. Whence, for instance, arose the institution of chattel slavery, so repugnant to our modern ideas of right and wrong, and how shall we explain its defense and justification in the name of religion and morality? How account for the fact that what Yesterday regarded as righteous, To-day condemns as wrong; that what at one period of the world's history is regarded as perfectly natural and right--the practice of polygamy, for example--becomes abhorrent at another period; or that what is regarded with horror and disgust in one part of the world is sanctioned by the ethical codes, and freely practiced elsewhere? Ferri gives two examples of this kind: the cannibalism of Central African tribes, and the killing of parents, as a religious duty, in Sumatra.[61] To reply "custom" is to beg the whole question, for customs do not exist without reason, however difficult it may be to discern the reason for any particular custom. To reply that these things are mysteries, as the old theologians did when the doctrine of the Trinity was questioned, is to leave the question unanswered and to challenge doubt and investigation. The human mind abhors a mystery as nature abhors a vacuum. Despite Spencer, the human mind has never admitted the existence of the _Unknowable_. To explore the _Unknown_ is man's universal impulse; and with each fresh discovery the _Unknown_ is narrowed by the