Let us go back to the simplest organism conceivable, which must have represented the beginning of life on our earth, and we see that this need not have possessed any special power of regeneration, because, for an organism without differentiation of parts, growth is equivalent to regeneration. But growth is the direct outcome of one of the primary characters of the living substance, the capacity of assimilation. This cannot be an adaptive phenomenon, nor can it have arisen through selection, because selection presupposes reproduction, and reproduction is only a periodic form of growth; but growth follows directly from assimilation. The fundamental characters of the living substance, above all the dissimilation and assimilation which condition metabolism, must have been in existence from the first when living substance arose, and must depend on its unique chemico-physical composition. But the faculty of regeneration could only be acquired when organisms became qualitatively differentiated, so that each part was no longer like every other part or like the whole. As soon as this stage was reached the faculty of regeneration would necessarily be developed, if further multiplication was to take place. For when each fragment could no longer become a whole by simply growing, some arrangement had to be made by which each fragment should receive, in the form of primary constituents, what it lacked to make up the whole. We do not know the first beginning of this adaptation, but, in its further development, it appears in the form of 'nuclear substance,' enclosed in the nucleus of the cell, and, as is well known, it is now to be found in all unicellular organisms. That the nucleus there precedes regeneration in the sense that without a piece of it the cell-soma is not able to complete itself alone, we have already seen, and the explanation of this fact has always seemed to me to be that invisibly minute vital units relating to the regeneration of the injured part leave the nucleus and evoke the development of the missing parts by laws and forces still unknown to us. Loeb has recently claimed that the nucleus is the cell's organ of oxidation; but if that be true it would still not exclude the possibility that the nucleus is also and primarily a storehouse of the material bearers of the primary constituents of a species. It must be regarded as such when we call to mind the phenomena of amphimixis in its twofold aspects as conjugation and as fertilization, and its obvious outcome among higher organisms where it implies the mingling of the parental qualities.