This theory, the importance of which to the further development of phytotomy and vegetable physiology has been already pointed out, will form the conclusion of our history of the anatomy of plants. It was a remarkable coincidence that this molecular theory of organic forms, which is not without results for zootomy also, was brought to completion at about the same time, namely, the year 1860, that Darwin first published his theory of descent. At the first glance the two theories seem to have no connection with one another, and so the coincidence in time appears to be quite accidental. But if we go deeper into the matter, we find a resemblance between them which is of great historical importance; they both of them exchange the purely formal consideration of organic bodies, which had prevailed up to that time, for a consideration of causes; as Darwin’s doctrine endeavours to account for the specific forms of animals and plants from the principles of inheritance and variability under the disturbing or favouring influence of external circumstances, so the object of Nägeli’s theory is to refer the growth and inner structure of organised bodies to chemical and mechanical processes. The future will show, whether the views which we owe to Nägeli will not contribute to the laying a deeper foundation for the theory of descent, since it is not improbable that a more thorough understanding of the molecular structure of organisms may add light and certainty to the still obscure conceptions of inheritance and variation.