The community of the phenomena of vitality in animals and plants being thus placed beyond a doubt, we must now discover the reason why. This reason is to be found in their anatomical and in their chemical unity. The fundamental phenomena are common because the composition is common, and because the universal anatomical basis, the cell, possesses in all cases a sum total of identical properties.
If we appeal to physiology for the characteristics common to living beings, it will generally give us the following:—A structure or organization; a certain chemical composition which is that of living matter; a specific form; an evolution which in the earliest stage occasions the being to grow and develop until it is divided, and which in the highest stage includes one or more evolutive cycles with growth, the adult stage, senility, and death; a property of increase or nutrition, with its consequence—namely, a relation of material exchanges with the ambient medium;—and finally, a property of reproduction. It is important to pass them rapidly in review.
