The theory of evolution, as I understand it, is founded on the supposition—or rather I think I may say the known fact—that the cells of a multicellular organism such as man correspond during the successive stages of the ontogeny, to the cells of the successive non-conjugating generations which intervene between one act of conjugation and the next among unicellular organisms. If this be admitted, it does not affect the argument that the number of cell-generations is in some cases almost infinitely prolonged,—e.g. among aphides,—nor even that new organisms in some instances arise from non-conjugating cells as among bees, or by budding as among the hydrozoa. The essential point is, that though the cells remain adherent and undergo differentiation and specialization, yet they are all cell-descendants of the germ cell, or the pair of conjugated germ cells, in precisely the same sense as the cell-descendants of a unicellular organism or conjugated pair of unicellular organisms are its or their cell-descendants. By the theory that inborn variations are alone transmissible, it is assumed that no single cell of a multicellular organism is in part or whole a product of any other co-existing cell or cells, i.e. that it has not received from any of them living constituents, but that it is merely a co-descendant with the other cells from a common ancestry; for instance, that the germ cells are not in whole or part products of the other co-existing cells, but are co-descendants with the other cells from a common ancestry, and do not receive from them living "gemmules," which become part of them, and so alter their constitution as to cause them to proliferate into organisms different from that which would otherwise have arisen. If this be true, if the germ cells are not even in part products of the other co-existing cells, and if it be also admitted that acquired variations cannot so alter the nutritive fluids as to produce similar variations in the new organisms
