Physiological chemistry has, after countless analyses, established the following five facts during the last forty years: I. No other elements are found in organic bodies than those of the inorganic world. II. The combinations of elements which are peculiar to organisms, and which are responsible for their vital phenomena, are compound protoplasmic substances, of the group of albuminates. III. Organic life itself is a chemico-physical process, based on the metabolism (or interchange of material) of these albuminates. IV. The only element which is capable of building up these compound albuminates, in combination with other elements (oxygen, hydrogen, nitrogen, and sulphur), is carbon. V. These protoplasmic compounds of carbon are distinguished from most other chemical combinations by their very intricate molecular structure, their instability, and their jelly-like consistency. On the basis of these five fundamental facts the following "carbon-theory" was erected thirty-three years ago: "The peculiar chemico-physical properties of carbon--especially the fluidity and the facility of decomposition of the most elaborate albuminoid compounds of carbon--are the sole and the mechanical causes of the specific phenomena of movement, which distinguish organic from inorganic substances, and which are called life, in the usual sense of the word" (see _The Natural History of Creation_). Although this "carbon-theory" is warmly disputed in some quarters, no better monistic theory has yet appeared to replace it. We have now a much better and more thorough knowledge of the physiological relations of cell-life, and of the chemistry and physics of the living protoplasm, than we had thirty-three years ago, and so it is possible to make a more confident and effective defence of the carbon-theory.