If we rightly consider this circumstance, and if we further reflect upon it in connection with the empirical foundation of the philosophical theory of development, we shall at once see how we are placed respecting the oft-demanded _proofs of the theory of descent_. The more the doctrine of filiation has of late years made way for itself, and the more all thoughtful, younger naturalists, and all truly biologically-educated philosophers have become convinced of its inner truth and absolute necessity, the louder have its opponents called for actual proofs. The same persons who, shortly after the publication of Darwin’s work, declared it to be “a groundless, fantastic system,” an “arbitrary speculation,” an “ingenious dream,” now kindly condescend to declare that the theory of descent certainly is a scientific “_hypothesis_,” but that it still requires to be “_proved_.” When these remarks are made by persons who do not possess the requisite empirico-philosophical culture, nor the necessary knowledge in comparative anatomy, embryology, and palæontology, we cannot be much offended, and we refer them to the study of those sciences. But when similar remarks are made by acknowledged specialists, by teachers of zoology and botany, who certainly ought to possess a general insight into the whole domain of their science, or who are actually familiar with the facts of those scientific domains, then we are really at a loss what to say. Those who are not satisfied with the treasures of our present empirical knowledge of nature as a basis on which to establish the Theory of Descent, will not be convinced by any other facts which may hereafter be discovered; for we can conceive no circumstances which would furnish stronger or a more complete testimony to the truth of the doctrine of filiation than is even now seen, for example, in the well-known facts of comparative anatomy and ontogeny. I must here again direct attention to the fact, _that all the great and general laws, and all the comprehensive series of phenomena of the most different domains of biology can only be explained and understood by the Theory of Development_ (and especially by its biological part, the Theory of Descent), and that without it they remain completely inexplicable and incomprehensible. _The internal causal connection_ between them all proves the Theory of Descent to be the greatest _inductive law_ of Biology. Before concluding, I will once more name all those series of inductions, all those general laws of Biology, upon which this comprehensive law of development is firmly based.