Fusion of this kind was demonstrated in the Myxomycetes almost forty years ago by De Bary, and it has been observed more recently in various unicellular organisms, especially in Rhizopods and in Heliozoa. These last often place themselves close together in pairs, threes, or even more at a time, and then the delicate cell-bodies coalesce, though no fusion of the nuclei takes place. With Hartog, we call this process 'Plastogamy,' but we cannot agree with that observer when he regards the importance of the process as consisting in the fact that the nuclei thus come into contact with fresh cell-substance, after having been surrounded for a very long time with the same cytoplasm. If this were the import of amphimixis, then an _exchange of nuclei_ would take place, and this we find nowhere even among the lowest forms of life, for everywhere there is a union of the nuclear substance of two individuals. But this is by the way! Further cases of plastogamy have been observed in many of the limy-shelled Rhizopods. A union of this kind does not usually lead to any visible consequences, but in some Foraminifera a group of young animals is developed within the cell-bodies by the division of the nuclei and the cell-body; thus multiplication follows the fusion just as in perfect amphimixis, and we may therefore assume that there is a causal connexion between the two. In the slime-fungi, too, the union of several amœba-like cells into a multi-nucleated plasmodium is followed later by the development of numerous encapsuled spores, but only after the plasmodium, which to begin with is microscopically small, has grown to a macroscopically visible, reticulated mass (_Æthalium_) sometimes a foot in extent. In this case the fungus, creeping slowly over its foundation of decaying substance, takes up nourishment from it, and it is not possible to tell whether the union of the amœbæ yields any further advantage than that of facilitating the spreading over large uneven surfaces, and through this, later, the development of large fruit-bodies. But in the case of the Foraminifera the plastogamy has obviously another effect, unknown and mysterious, which as yet no one has ever been able to define precisely. Words like 'stimulus to growth,' 'stimulating of the metabolism,' and even 'rejuvenation,' give no insight into what happens, but that something happens, that through the fusion of two or more unicellulars a stimulus is exerted, which reveals itself later in increased rapidity of growth, we may, and indeed must assume, because this process has become a permanent arrangement in so many unicellular organisms. Only what is useful survives, and the uniting individuals must derive some advantage from the process of fusion, and it remains to be seen whether we can find out with any clearness what this advantage may be.