We certainly ought not to conclude from the fact that we cannot see into the structure and requirements and methods of nutrition of a very minute mass of substance that its nutrition cannot be unequal, and that it cannot, by its inequalities, give rise to very material differences, especially when we are dealing with a substance to which we must attribute an extraordinarily complex organization built up of enormous numbers of extremely minute particles. That this complexity is undeniable is now admitted by many who formerly thought it possible to believe in the simple structure of the germ-substance. How complex not only the germ-substance but every cell of a higher organism is in its structure, and how far below the limits of visibility its differentiations and arrangements reach, is pressed upon our attention by the most recent histological researches, such as those we owe to Heidenhain, Boveri, and many others. The whole scientific world was amazed when it came to know the mysterious nuclear spindle in the seventies, and since then this has been quite thrown into the shade by the discovery of the centrosphere, the centrosome, and more recently even the centriole, and now we believe that these marvellous centres of force may, or must, possess their own dividing apparatus! In the face of discoveries like these no one is likely to be able to persist in recognizing as existing only what is disclosed or even hinted at by the most powerful lenses; no one can any longer doubt that far below the limit of visibility organization is still at the basis of life, and that it is dominated by orderly forces. To me, at least, it seems more cogent to argue from the phenomena of heredity and variation to an enormous mass of minute vital units crowded together in the narrow space of the id, than to argue from the calculated size of atoms and molecules to the number which we are justified in assuming to be present in an id. In my book on the germ-plasm I made a calculation of this kind, and I arrived at figures which seemed rather too small for the requirements of the germ-plasm theory. This has been regarded as a proof that I disregard the facts for the sake of my theory, but it should rather be asked whether the size of the atoms and molecules is a fact, and not rather the very questionable result of an uncertain method of calculation. Undoubtedly modern chemistry has established the _relative_ weight-proportions of the atoms and molecules with admirable precision, but it can make only very uncertain statements in regard to the _absolute_ size of the ultimate particles. It is therefore admissible to assume that these have a still greater degree of minuteness when the facts in another domain of science require this.