CHAPTER IV. EXAMINATION OF WEISMANN’S THEORY OF EVOLUTION (1891). HAVING now considered germ-plasm as perpetually continuous, we have next to regard it as unalterably stable. First, let it be noted that these two fundamental and distinctive postulates of the whole Weismannian system are so intimately connected as to be in large measure mutually dependent. For, on the one hand, if germ-plasm has not been perpetually continuous since the first origin of life, it cannot have been absolutely stable “since the first origin of sexual propagation”: every time that its hereditary characters are modified by its containing soma (whether or not representatively so), its stability has been so far upset. On the other hand, if germ-plasm has not been absolutely stable, it cannot have been perpetually continuous “since the first origin of life.” As often as its stability has been upset, its “molecular structure” has been modified by causes _ab extra_, as distinguished from mixtures of germ-plasms in sexual unions. Therefore, it can no longer have been continuous in the sense of having borne an ineffaceable record of all congenital variations, _due to sexual unions_, throughout the entire phylogeny of the Metaphyta and Metazoa. At most it can have been continuous only in the attenuated sense, that however much and however often its hereditary characters may have been modified by somatic changes on the one hand or by changes in the external conditions of life on the other, they can never have been thus modified _representatively_, as supposed by the theory of pangenesis. From which it follows that, while examining in our last chapter Weismann’s doctrine of the perpetual continuity of germ-plasm, we have been indirectly examining also his companion doctrine of the unalterable stability of germ-plasm. Nevertheless, for the sake of doing justice to both these doctrines, I have thought it desirable to examine each on its own merits, without prejudice arising from our criticism of the other. To such a separate and independent examination of the doctrine of unalterable stability we will, therefore, now proceed. * * * * *