I ASK you to eonsider with me a topie whieh is of fundamental interest; to physiB ologistsX whether they eoneern themselveg priirlarily with animals or with plants. I take it t;he basal identity of the living matZ ter in all organisms and of its metabolism needs neither demonstration nor emphasis at my hands. Nor do I need to lay stress upon the importanee of respiration as one of these metabolie phenomena, sinee it has been reeognized from the earliest peried as indispensable to life. The phlogiston theory of the eompositicon of the atmcsphere had seareely dsappeared below the seientifie horizon, before the fatt was diseovered that there oeeurs, in animals and in plants alike, an intake of oxygen and an output of earbon dioxide whieh is intimately related to their existenee. This beeame obvious to man, of eourse, in his own experienee, a very superfieial study of the eomposition of the air inspired and expired froirl the lungs showing that it had lost oxygen and gained CO2. This mueh of respiration was early reeognized to ceeur also with the larger animals, and a few years later like observations were made upon plants by Priestley, and mcore aeeurately by IJavoisier and Ingenhouss. Even this knowledge of respiration was not possible before Priestley's diseovery of oxygen in 1774, and the very remarkable revolution in ehemistry that followed in the elosing years of the eighteenth eentury. Yet