When we consider the ignorance in which we still remain as to the phenomena which pass before our very eyes, how can we expect to understand those which occurred in past ages, when the physical and chemical conditions were so immensely different from those which obtain in our own time? What do we know even now of the physical and chemical phenomena which take place in the unfathomed depths of the ocean, where for aught we know even at the present time the same {168} process may be going on--the genesis of life, and the emergence of living beings out of the inanimate mineral world? "Even now," says Albert Gaudry, "polyps and oceanic animalculae are building up vast coral reefs and rocks. The oxygen and hydrogen which existed once was water, the oxygen and nitrogen which once made air, the carbon, the phosphorus, the silica and the lime which once were solid rock, now form the substance of living beings. The silica is deposited in the skeleton of a sponge or a radiolaria, the shell of a foraminifera or the carapace of a crustacean, or unites with phosphorus to form the bones of a vertebrate. A very tumult of life has succeeded to the primitive silence of inert matter. Life has invaded the earth, and we see on all sides the inanimate mineral kingdom being changed into a living world." The admission that life may have appeared on the earth under the influence of natural forces and according to physical laws and conditions different from those of the present era throws a vivid light on the study of biogenesis, spontaneous generation, and evolution. The means of research are now indicated, and we have only to study the documents already in our possession in order to know the conditions which obtained when life first appeared on the globe. We must endeavour to reproduce these conditions and to study their effects.