The very sciences that deal with these different modes of matter and motion are now by no means as clearly marked off one from another as their earlier students thought they were. Physics, chemistry; geology, botany, zoology, anatomy, physiology, how they all dovetail into, or actually overlap each other. It is impossible to say sometimes to which domain of science a particular fact belongs. The distinctions between the physical and the chemical properties of bodies are confessedly artificial. Botany implies a study of the anatomy and the physiology of plants. Physiology in its turn becomes only a question of chemistry; its phenomena are becoming reduced to mathematical expressions. We are learning to calculate the actual amount of work done in the performance of different functions of the living body, in the same terms as we calculate the work done by a steam engine. The respiratory organs or the muscular during the day do so many foot-pounds of work. The foot-pound is the unit of measurement employed in the study of work. Work is done when matter is moved through space. The footpound is the amount of work done when the mass of a pound is raised one foot against the gravitation attraction of the earth. A steam-engine does per day a certain number of foot-pounds of work. Its capacity for work is usually expressed by saying that it is so many horse power. One horse power is equivalent to 33,000 footpounds per minute. The physiologists are, by means of very intricate and careful calculations, enabled to calculate with ever-increasing accuracy the equivalent in footpounds, i. e., the mechanical equivalent, of each of the body functions of the average man per diem.
If we turn to any of the special sciences the same dovetailing and over-lapping appear. In chemistry it is difficult to mark off any group of bodies from all other groups. The three sets of bodies that chemistry is supposed to study are elements, mixtures, and compounds. An element such as carbon or gold, is a body which has not yet been decomposed. A mixture is that which results from putting together two or more substances, without those substances undergoing any change of properties. Thus brandy and water, or gunpowder is a mixture. The properties of the brandy and of the water in the one case, and of the charcoal, nitre and sulphur in the other, are unchanged. A compound is the result of the union of two or more elements with change of properties; thus water is a compound of hydrogen and oxygen, and its properties are those of neither hydrogen nor oxygen. The fundamental distinction supposed to be at the basis of all chemical study, that between elements and compounds, is found to be inapplicable when we study such bodies as cyanogen, a compound of the two elements carbon and nitrogen, that behaves like an element. Ammonium, a compound of four atoms of hydrogen and one of nitrogen, also behaves like an element, taking the place of such metallic elements as potassium or sodium. In fact all the so-called "compound radicles" which enter so largely into our study of organic chemistry are groups of two or atoms of two or more elements that behave as simple bodies. The metals and the non-metals are connected by such forms as arsenic or selenium, placed by one chemist among the metals, by another among the non-metals. Hydrogen, usually classed with the non-metals, has the power of replacing metallic elements. It does this so persistently that, on theoretical grounds, chemists had long spoken of hydrogen as probably essentially a metal. When the French chemist Pictet succeeded in liquefying hydrogen, until then only known in the gas form, the liquid fell upon the floor of the laboratory with a metallic ring. And who is to say positively whether an alloy of copper and zinc is to be regarded as a mixture or as a compound of the two metals?
Still more important is the bridging over the supposed gulf between the inorganic and the organic chemical substances. A few years back this gulf was supposed to be great, fixed, impassable. The mineral or inorganic was makable by man. The organic was not, and never would be. The chemist might go on continually manufacturing hydrogen and oxygen, carbon dioxide, ammonia. But he was never to hope to make alcohol, sugar, urea, any of the multitudinous substances called organic. And now all this folly of forbidding is at an end. The organic bodies are manufactured by man. The inorganic and the organic are no more regarded as clearly distinguishable. Even the chemistry books by their very titles recognise and proclaim this fact. We have no longer works on organic chemistry. We have volumes on the chemistry of carbon compounds.
In geology the different kinds of rocks graduate into each other. Between the aqueous, or sedimentary, and the igneous, or those due to the action of fire, range the metamorphic, i.e., sedimentary rocks that have been afterwards subjected to heat. The various systems of sedimentary rocks are known now to be purely artificial if convenient divisions. From the Laurentian up to the recent rocks there has never been any real hiatus. Nowhere is there the slightest evidence of pause or of recommencement. Our groups are artificial. Nature is like Oallio and cares for none of these things.
Whilst rocks thus glide one into the other, the fossil remains that they contain do likewise. If the view of the special creationist were accurate we ought to find isolated forms of dead animals and plants, we ought to find sudden appearances in the rocks of forms not allied to these already encountered, we ought not necessarily to find a series of organic remains ascending in complexity of structure. If the view of the evolutionist is accurate, we ought to find no forms of dead animals or plants isolated; we ought never to find a form appearing without preliminary heralds of its coming in the shape of kindred forms; we ought to find a series of organic remains whose later members are in advance of the earlier. These latter expectations are realised.