I have already shown in the fourteenth chapter of the _Riddle_(and at greater length in the fifteenth chapter of my _History of Creation_) that the belief in the essential unity of nature, or the monism of the cosmos, is of the greatest importance for our whole system. I gave a very thorough justification of this cosmic monism in 1866. In the fifth chapter of the _Generelle Morphologie_ I considered the relation of the organic to the inorganic in every respect, pointing out the differences between them on the one hand, and their points of agreement in matter, form, and force on the other. Nägeli some time afterwards declared similarly for the unity of nature in his able _Mechanisch-physiologische Begründung_ _der Abstammungslehre_(1884). Wilhelm Ostwald has recently done the same, from the monistic point of view of his system of energy, in his _Naturphilosophie_, especially in the sixteenth chapter. Without being acquainted with my earlier work, he has impartially compared the physico-chemical processes in the organic and inorganic worlds, partly adducing the same illustrations from the instructive field of crystallization. He came to the same monistic conclusions that I reached thirty-six years ago. As most biologists continue to ignore them, and as, especially, modern vitalism thrusts these inconvenient facts out of sight, I will give a brief summary once more of the chief points as regards the matter, form, and forces of bodies. Chemical analysis shows that there are no elements present in organisms that are not found in inorganic bodies. The number of elements that cannot be further analyzed is now put at seventy-eight; but of these only the five organogenetic elements already mentioned which combine to form plasm--carbon, oxygen, hydrogen, nitrogen, and sulphur--are found invariably in living things. With these are generally (but not always) associated five other elements--phosphor, potassium, calcium, magnesium, and iron. Other elements may also be found in organisms; but there is not a single biological element that is not also found in the inorganic world. Hence the distinctive features which separate the one from the other can be sought only in some special form of combination of the elements. And it is carbon especially, the chief organic element, that by its peculiar affinity enters into the most diverse and complicated combinations with other elements, and produces the most important of all substances, the albuminoids, at the head of which is the living plasm (_cf._ chapter vi.).