Lastly, and as regards the multicellular organisms, it is evident that Weismann’s essay _On the Significance of Sexual Reproduction in the Theory of Natural Selection_ must be cancelled. For, apart from the contradictory manner in which this matter has been stated (pp. 70, 93, notes), and apart also from the consideration that other and quite as probable reasons have been suggested for the origin of sexual reproduction, there is the fact that Weismann’s theory is no longer tenable after the above destruction of its logical postulate in the absolute stability of germ-plasm. For, in the absence of this postulate, there is no basis for the theory that admixtures of germ-plasms in sexual reproduction furnish the sole means whereby heritable variations can be supplied for the working of natural selection. _Summary._ THE theory of germ-plasm is not only a theory of heredity: it is also, and more distinctively, a theory of evolution. As a theory of heredity it is grounded on its author’s fundamental postulate—the continuity of germ-plasm; and, further, on a fact well recognized by all other theories of heredity, which he expresses by the term stability of germ-plasm. But as a theory of evolution it requires two additional postulates for its support—viz., that germ-plasm has been _perpetually_ continuous “since the first origin of life,” and _absolutely_ stable “since the first origin of sexual reproduction.” It is clear that these two additional postulates are not needed for his theory of heredity, but only for his additional theory of evolution. There have been other theories of heredity, prior to this one, which, like it, have been founded on the postulate of “continuity” (in Weismann’s sense) of the substance of heredity; but it has not been needful for any of these theories to postulate further that this substance has been _always_ thus isolated, or even that it is now _invariably_ so. For even though the isolation be frequently invaded by influences of body-changes on the congenital characters of this substance, it does not follow that the body-changes must be transmitted to offspring exactly as they occurred in parents. They may produce in offspring what we have agreed to call “specialized” hereditary changes, even if they never produce “representative” hereditary changes,—i.e., the transmission of acquired characters. But it is essential to Weismann’s theory of _evolution_ that body-changes should not exercise a modifying influence of _any_ kind on the ancestral endowments of this substance; hence, for the purposes of this further theory he has to assume that germ-plasm presents, not only _continuity_, but continuity _unbroken since the first origin of life_.