of preformation was attracting wide interest. This had stimulated Caspar Friedrick Wolff to the production of his Theoria Generationis, which was unfortunately held in the dark by the opposition of Haller who could not accept the principles which led, at a somewhat later date, to the conception of Epigenesis. The theory of irritability was also a bone of contention, and though it was materially furthered toward the true conception by Haller's own researches, these last, unfortunately, served also to further a doctrine which thoroughly permeated and confused the development of all physiology down to the middle of the nineteenth century. This was due chiefly to the following fact: That the seeming impossibility of explaining the phenomena of irritability led to the welcoming of the theory of vitalism, or vital force, which asserted a distinct dualism between living and lifeless nature. The vitalists at this time (and nearly all the natural scientists, except perhaps Rudolphi at Berlin, were vitalists in a greater or lesser degree) were discarding the mechanical and chemical explanation of life phenomena, and were introducing such mysterious and inscrutable explanatory principles as la force hypermécanique and the nisus formativus. In this acute and exhaustive manner were explained even the most complex of vital phenomena.
Toward the end of the eighteenth century, however, some twenty years before the birth of Müller, a new note was being sounded from the ranks of German scientists, especially from Reil, whom we may well call the censor of German vitalism. In his work "Ueber die Seelenskraft," he was forcing upon unwilling hearers not only the conception that the life phenomena of living organisms are regulated by chemico-physical laws, but that there were higher principles in control which were present only in living matter. The few adherents to the chemico-physical hypothesis were, during the last years of the eighteenth century, receiving fundamental support from such men as Ritter, Galvani and Humboldt. Through the work of these men the notion was becoming popular that the galvanic current was the cause of all vital phenomena.
Among the chemical and physical discoveries of this time we can mention the advance of vegetable physiology through Ingenhaus (1730-99), who developed the theory of the consumption of carbon dioxide by plants; the discovery of oxygen by Priestley (1733-1804) and Lavoisier (1743-94), and the further discovery in this line by Girtanner, who showed that the venous blood is aerated in the lungs. Thus the existence of the mystical "pneuma," which had clung with a peculiar persistence to centuries of physiological thought, had now become a reality. The anatomical researches of this period were characterized by one discovery in particular, announced by Charles Bell in 1810; that is, the fundamental law of specific nerve physiology, to be later experimentally proved by Johannes Müller. In microscopy,
