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.
As I have already said in an earlier lecture, Darwin adhered to Lamarck's assumption of the transmission of functional adaptations, and perhaps the easiest way to make clear the theoretical difficulties which stand in the way of such an assumption is to show how Darwin sought to present this principle as theoretically conceivable and possible.
Darwin was the first to think out a theory of heredity which was worthy of the name of theory, for it was not merely an idea hastily suggested, but an attempt, though only in outline, at elaborating a definite hypothesis. His theory of 'Pangenesis' assumes that cells give rise to special gemmules which are infinitesimally minute, and of which each cell brings forth countless hosts in the course of its existence. Each of these gemmules can give rise to a cell similar to the one in which it was itself produced, but it cannot do this at all times, but only under definite circumstances, namely, when it reaches 'those cells which precede in order of development' those that it has to give rise to. Darwin calls this the 'elective affinity' of each gemmule for this particular kind of cell. Thus, from the beginning of development there arises in every cell a host of gemmules, each of which virtually represents a specific cell. These gemmules, however, do not remain where they originated, but migrate from their place of origin into the blood-stream, and are carried by it in myriads to all parts of the body. Thus they reach also the ovaries and testes and the germ-cells lying within these, penetrate into them, and there accumulate, so that the germ-cells, in the course of life, come to contain gemmules from all the kinds of cells which have appeared in the organism, and, at the same time, all the variations which any part may have undergone, whether due to external or internal influences, or through use and disuse.
In this manner Darwin sought to attribute to the germ-cells the power of giving rise, in the course of their development, to the same variations as the individual had acquired during its lifetime in consequence of external conditions or functional influences.