[15] Delage, in _La structure du protoplasma et les theories sur l'heredite_, etc., Paris, 1895, is mistaken in attributing to Herbert Spencer the merit of having first pointed out the necessity of the assumption of biological units ranking between the molecule and the cell. Bruecke set forth this idea three years previously to Spencer and established it exhaustively in a paper which in Germany at least is famous ("Elementarorganismen," _Wiener Sitzungsberichte_, October 10, 1861, Vol. XLIV., II., p. 381). Spencer's _Principles of Biology_ appeared between 1864 and 1868; consequently there can be no dispute touching the priority of the idea. Strangely enough Delage cites Bruecke's essay in the Bibliographical Index at the end of his book correctly, although Bruecke's name and views are nowhere mentioned in the book itself. It is to be observed, however, that the elementary organisms of Bruecke are not merely the precursors of Spencer's "physiological units," but repose on much firmer foundations than the latter, which, as Delage himself remarks, are at bottom nothing more than magnified molecules and not combinations of different molecules of such character as to produce necessarily phenomena of life. He aptly remarks on this point: "the physiological units of Spencer are only chemical molecules of greater complexity than the rest, and as he defines them they would be regarded as such by every chemist. He attributes to them no property _essentially_ different from those of chemical molecules." Assimilation, growth, propagation, in short the attributes of life, are not attributed by Spencer to his units, while Bruecke by his very designation "elementary organisms" expresses the idea of "ultimate living units," to use Wiesner's phrase. Of course this particular aspect of the vital units was not emphasised by Bruecke with the same distinctness and sharpness as by recent inquirers, who took up Bruecke's ideas thirty years after. I refer to the conception that the union of a definite combination of heterogeneous molecules into an invisibly small unit, forms the cradle or focus of the vital phenomena. This was first done and apparently on independent considerations by De Vries, and soon after by Wiesner, and subsequently by myself (De Vries, _Intracellulaere Pangenesis_, Jena, 1889; Wiesner, _Die Elementarstructur and das Wachsthum der lebenden Substanz_, Vienna, 1892; Weismann, _Das Keimplasma_, Jena, 1892). Let me say at the close of this note that it is not my intention in thus defending the rights of a great physiologist, to censure in the least the distinguished author of _L'heredite_ who has set himself a remarkably high standard of exactitude in such matters. Certainly, when we consider the enormous extent of the literature that had to be mastered to produce his book, embracing as it did all the various theories of recent times, such an oversight is quite excusable.