Heredity, the conservative principle in inheritance, or the tendency of organisms to resemble their parents, is practically at present one of the ultimate or unexplained facts of biology. Hypothetical explanations have been suggested by Darwin (q.v.) in his theory of Pangenesis (q.v.), and by Professor Weismann in that of the "permanence of the germ-plasm." Some unexplained generalisations of the facts of heredity can be stated as "laws." Among these are the law of ontogenetic recapitulation of phylogeny, formulated by Von Baer (q.v.), that each individual in its development passes through a series of forms representing its ancestors of a less and less remoteness; that of precocity or anticipatory inheritance, pointed, out by Darwin, by which characters tend to be inherited at slightly earlier stages of development; and that of atavism or reversion, by which characters sometimes skip one or more generations, offspring resembling grandparents more than parents. Another remarkable class of cases is the inheritance of certain characters exclusively by one sex among the offspring. The inheritance of acquired characters is denied by Weismann, but this question is still under discussion. The strength of this principle, even in the field of moral character, has of late been made the foundation of a literature which dangerously undermines the doctrine of moral responsibility.