We know that upon entering the world the human being springs from two sources. His physical body is inherited from his ancestors, from his father and mother and their forbears. He inherits certain traits, good or bad, that are simply inherent in the blood, in the line of descent. But in every case of this sort the forces a child brings along from his previous incarnation unite with these inherited qualities. Now, you know that today a great deal is talked about “hereditary tendencies” whenever some disease or other makes its appearance. How this term is abused nowadays — though it is quite justified within a narrow scope!
Whenever anything crops up that can be proved to have been an attribute of some ancestor, hereditary tendencies are invoked; and because people know nothing of active spiritual forces derived from the previous incarnation they endow these inherited tendencies with overwhelming power. If they knew that a spiritual factor accompanied us from our previous incarnation they would say, Well and good: we believe absolutely in hereditary tendencies, but we know as well what stems from the previous incarnation in the way of inner, central soul forces, and that if sufficiently strengthened and invigorated these will gain the upper hand over matter — that is, over hereditary tendencies.
— And such a man, capable of rising to the cognition of spirit, would continue: No matter how powerfully the inherited tendencies affect me, I shall provide nourishment for the spirit in me; for in this way I shall master them. — But anyone who does not work upon his spiritual nature, upon that which is not inherited, will positively fall a prey to inherited tendencies as a result of such lack of faith; and in this way materialistic superstition will actually bring about a steady increase in their power over us. We shall be engulfed in the quagmire of hereditary tendencies unless we fortify our spirit and, by means of a strong spirit, vanquish each time anew whatever is inherited.
Source: Rudolf Steiner – GA 112 – THE GOSPEL OF ST. JOHN IN RELATION TO THE OTHER THREE GOSPELS – Kassel, 30 June 1909