I am inclined to regard the separation of parental traits in the progeny of hybrids in Pisum as complete and thus permanent. The progeny of hybrids carries one or the other of the parental traits, or the hybrid form of the two; I have never observed gradual transitions between the parental traits or a progressive approach towards one of them. The course of development consists simply in this; that in each generation the two parental traits appear, separated and unchanged, and there is nothing to indicate that one of them has either inherited or taken over anything from the other

April 18, 1867. - Letters to Carl Nägeli