development of molecules from atoms, and if the theory of light waves should not be correct we have no absolute knowledge that we even see these interesting things. The lapse of time produces a very thin crop of final truths of last instance. In geology we are in a still more embarrassing situation for we are here involved in the study of preceding epochs in which, as a matter of fact, neither we ourselves nor any other human being ever existed. Here there is much labor spent in the harvesting of truths of last instance, and they are a scanty crop withal. The second division of knowledge is occupied in the investigation of living organisms. In this field the changes and causalities are so complex that not only does the solution of each question bring about the rise of an unlimited number of new questions, but the solution of each of these separate new questions depends upon years, frequently centuries, of investigation, and can then be only partially completed. So that the need of systematic arrangement of the various interrelations continually surrounds the final truths of the last instance with a prolific and spreading growth of hypotheses. Look at the long succession of progressive steps from Galen to Malpighi necessary to establish correctly so simple a thing as the circulation of the blood of mammals, yet how little we know of the origin of blood corpuscles and how many mistakes we make in, for example, rationally connecting the symptoms and cause of a disease. Besides there are frequently discoveries like those of the cell which compel us to entirely revise all hitherto firmly established truth of the last instance in biology, and to lay numbers of such truths aside for good and all. He who would therefore in this science undertake the proclamation of absolute and immutable truths must be content with such platitudes as the following: "All men must die; all female mammals have mammary glands, etc." He will not even be able to say that the greater animals digest their food by means of the stomach and bowels and not with the head because the centralised system of nerves in the head is not adapted to digestion.