Thus the riddle of Xenia is essentially solved. We understand how paternal primary constituents may find their way into the endosperm, and indeed must do so regularly; we understand also how the paternal influence never goes beyond the endosperm. The riddle is thus not only solved, but at the same time the view which assumes a fixed germ-plasm, and believes it to lie in the nuclear substance of the germ-cells, receives further confirmation, if it should need any, for if facts which are apparently contradictory to a theory can be naturally brought into harmony with it, this affords a stronger argument for the correctness of the theory than the power of explaining the facts which were used in building it up. There is much more to be said in regard to Xenia, and I am sure that much that is of interest will be brought to light by deeper investigation; theoretical difficulties will still have to be overcome, and I have already pointed out one of these in my '_Germ-plasm_,' but I must here rest satisfied with what has been already said. We have now passed in review and attempted to fit into the theory a sufficiently large number of the phenomena of heredity for the purpose of these lectures. Although, as is natural, much of this must remain hypothetical, we may accept the following series of propositions as well founded: there is a hereditary substance, the germ-plasm; it is contained in very minimal quantity in the germ-cells, and there in the chromosomes of the nucleus; it consists of primary constituents or determinants, which in their diverse arrangement beside or upon one another form an extremely complex structure, the id. Ids and determinants are living vital unities. Each nucleus contains several, often many, ids, and the number of ids varies with the species and is constant for each. The ids of the germ-plasm of each species have had a historical development, and are derived from the germ-plasm of the preceding lineage of species; therefore ids can never arise anew but only through multiplication of already existing ids. And now, equipped with this knowledge, let us return to the point from which we started, and inquire whether the Lamarckian principle of evolution, the inheritance of functional modifications, must be accepted or rejected. LECTURE XXIII EXAMINATION OF THE HYPOTHESIS OF THE TRANSMISSIBILITY OF FUNCTIONAL MODIFICATIONS Darwin's Pangenesis--Alleged proofs of functional inheritance--Mutilations not transmissible--Brown-Séquard's experiments on Epilepsy in guinea-pigs--Confusion of infection of the germ with inheritance, Pebrine, Syphilis, and Alcoholism--Does the interpretation of the facts require the assumption of the transmission of functional modifications?--Origin of instincts--The untaught pointer--Vom Rath's and Morgan's views--Attachment of the dog to his master--Fearlessness of sea-birds and seals on lonely islands--Flies and butterflies--Instincts exercised only once in the course of a lifetime.