Animals and plants derive their nourishment from different sources. The pabulum of the vegetable kingdom is derived from the inorganic world in the form of water, carbonic acid, and ammonia. Animals, on the other hand, are unable to assimilate these simple compounds, and can only live on protoplasm already prepared for them, either by vegetables or by other animals, which have, in their turn, absorbed previous vegetable protoplasm into their own bodies. Again animal and vegetable chemistry are, as it were, essentially antagonistic. The chemistry of vegetable life is synthetic, it takes simple compounds and, after rejecting those portions it does not require, builds of the remainder compound substances of great complexity. Animal chemical processes are analytic; they consist in seizing these highly complex matters, and reducing them to the simple compounds in which they originally existed. From this we learn that the essential element of life consists in the eternal and incessant circulation of matter. The vegetable kingdom takes water, carbonic acid, and ammonia, separates and discharges the oxygen it does not require, for the use of the animal kingdom, forms complex compounds of the remainder—protoplasm, vegetable albumen, gluten, starches, oils, fats, sugars for food, those volatile oils to which the scent of flowers is due, resins, camphors, guttapercha, turpentines, india-rubber, alkaloids as quinine, morphine, strychnine, and many others; indeed, the number of these vegetable products is infinite. Again, animals seize upon the oxygen exhaled by plants, and convert it into carbonic acid. They feed upon the protoplasm provided for them by the vegetable kingdom, and after utilizing it for the higher functions of animal life—locomotion, consciousness, sensation, thought, reason—they return it to the inorganic kingdom as water, carbonic acid, and ammonia, to be again taken up by vegetables, and recommence the never ending cycle of physical and chemical change. Letourneau remarks, “In living beings, in effect, matter is in a state of extreme mobility; it is subject to a perpetual movement of combination and decombination, without repose, without truce; its elements go and come, have reciprocities of action, aggregate themselves, disaggregate themselves; there is a whirl of atoms amongst unstable compounds, capable of forming, disaggregating, metamorphosing themselves, of renewing the woof of the living tissues.” And Professor Huxley tells us that, “the wonderful noon-day silence of the tropical forests is, after all, only due to the dullness of our hearing; and could our ears catch the murmur of the molecules as they whirl in the innumerable myriads of living cells which constitute each tree, we should be deafened as with the roar of a mighty city.” If you have followed me hitherto you will see that all the physical and chemical phenomena of life which I have endeavoured to describe are purely