Most if not all of them proceed on the assumption, shown in § 66 to be needful, that the structural characters of organisms are determined by the special natures of units which are intermediate between the chemical units and the morphological units--between the invisible molecules of proteid-substances and the visible tissue-components called cells. Four years after the first edition of this volume was published, appeared Mr. Darwin's work, _The Variation of Animals and Plants under Domestication_; and in this he set forth his doctrine of Pangenesis. Referring to the doctrine of physiological units which the preceding chapters work out, he at first expressed a doubt whether his own was or was not the same, but finally concluded that it was different. He was right in so concluding. Throughout my argument the implication everywhere is that the physiological units are all of one kind; whereas Mr. Darwin regards his component units, or "gemmules," as being of innumerable unlike kinds. He supposes that every cell of every tissue gives off gemmules special to itself, and capable of developing into similar cells. We may here, in passing, note that this view implies a fundamental distinction between unicellular organisms and the component cells of multicellular organisms, which are otherwise homologous with them. For while in their essential structures, their essential internal changes, and their essential processes of division, the _Protozoa_ and the component units of the _Metazoa_ are alike, the doctrine of Pangenesis implies that though the units when separate do not give off invisible gemmules the grouped units do.