Here in the domain of physiological chemistry there come to light processes, reactions, which make a mock of all the rules and laws got from reagent tubes. Here in the living organism it is found that the “strongest” acid--sulphuric acid--is crowded out of its combinations by the “weakest”--carbonic acid; which means nothing else but that the concept of “strength” as it has been taken over from inorganic nature does not apply here at all. By reason of such experiences it has been found necessary to introduce a new concept, that of “avidity”; in other words, here as everywhere, one hobbles along at the heels of the facts of actuality, being obliged ever and again to adapt oneself to them anew as best one may. Here in the living organism, albumen, fats, and carbohydrates are worked up at temperatures at which they undergo no change under the action of the oxygen of the atmosphere. The most marvellous thing of all, however, is the action of the glands, which, in taking up the material to be elaborated, display a power of choice that, so far as our ideas go, defies all explanation. Not the least regard is here paid to chemical and physical laws as abstracted by science from inorganic nature. Complete arbitrariness prevails. The epithelium of the stomach, for example, possesses the power of always despatching the hydric chloride set free from sodium chloride in one direction--namely, into the excretory ducts of the rennet glands, and of always sending the sodium carbonate formed in another direction, back into the lymph and blood circulation. Examples such as this might be multiplied to almost any extent, did we here aim at completeness. The key-word to it all, as revealed to us by the latest researches in physiological chemistry, is--arbitrariness! Of course, as everywhere so also here, only give her time enough and science will come round to adjustments in thought, and with that to the formulation of all such facts into laws. In respect of such facts, however, it must clearly be understood that the purely mechanical view is no longer able to hold the field; that the teleological view has broken through the artificial embankments of the mechanical view and again poured forth over the level lands of scientific thinking. That which has hitherto given such weight to the mechanical view in physiology is the possibility, up to a certain degree, of reading the physiological facts mechanically. One can “read” the eye so far as its external apparatus is concerned, according to the laws of catoptrics and dioptrics; but the bearing of this upon the faculty of seeing or upon an explanation of that faculty is simply nothing. This is not the fitting place to deal with the revolting outrage upon sound thinking of which the scientific theory of vision is guilty in its interpretation of the reversed retinal image: that demands a chapter to itself.