The reasons why we regard matter and motion as all-sufficient in the explanation of all the phenomena of the universe are several. In the first place, no destruction of matter has ever been witnessed. Second, no destruction of motion, has ever been witnessed. The creation of either matter or motion has been equally unseen. Transformations of matter from one of its infinitely many forms to some other are constantly visible, and they are always unattended by the smallest increase or diminution in the actual quantity of matter. So also with motion – transformations without any change in quantity are continually occurring.
Thus, we see the rocks disintegrated by the action of rain and running water, "weathered" by the action of the air. We see the matter of which they consisted worn away and carried down by streams and rivers to be deposited at the mouths of rivers or on the beds of seas. Or we set fire to a candle and watch its matter combining with the matter of the air to form the products of combustion, carbon, dioxide, steam, and their fellows. Or a dead animal or plant is seen to decay slowly into these same gases that the burning candle gives forth and into certain inorganic salts. And these are all cases of the transformation of matter without any creation or destruction.
Or we see the molar motion of a student's hands bringing together some acid and two metals. At once chemical action, a form of molecular motion, is set up. The molar motion of hands, a piece of silk, and a glass rod results in electricity, a mode of molecular motion. Or we apply heat, a mode of molecular motion, to a bar of metal which expands, to a mixture of hydrogen and oxygen which unite chemically. Or to a crystal of tourmaline, one end of which becomes positively electric, the other negatively. These are all cases of the transformation of motion without any creation or destruction. In all these cases the amount of matter and the amount of motion remain unchanged. Only the quantities of particular kinds vary. The generalisation that the quantity of matter and motion in the universe is the same yesterday, to-day, and for ever, appears to be thoroughly established.
More than this. Not only is there no scientific basis whatever for the fancy of a creation or of a destruction of matter or of motion. The fancy is unthinkable. No human mind is capable of picturing to itself the passage from the material to the immaterial, the moment of time in which the non-universe began to be the universe.
Yet again. Up to the present time every explanation of every phenomenon of the universe has been in terms of matter and of motion. The law of gravitation, Kepler's three great generalisations in astronomy, the phenomena of attraction and repulsion in electrified and magnetised bodies, the nature of chemical elements and compounds, the relation between, plants and animals in regard to their effect on the air, the principles of variation, of natural selection, of heredity, of adaptation – these and thousands of other truths that unseal our eyes to the beautiful meaning of nature, are all explanations as to how certain forms of matter are in certain states of motion. And if up to the present hour all the explanations that have been forthcoming of natural things are in terms of the natural, we are entitled to conclude that all explanations hereafter will be in kindred terms.
Or we may look on the question in another way. In the days of man's greater ignorance everything was primarily or ultimately referred to the supernatural. All phenomena were at first directly due to the action of the supernatural. But, as time and knowledge advanced, these references grew fewer and fewer in number. They were replaced by perfectly natural explanations of events, and we are entitled to believe that this process of elimination has now gone on sufficiently far for us to hold that since super-naturalism is unnecessary for the primary explanation of phenomena, it is also unnecessary for their ultimate explanation.
From all that I have just said it will be understood that the Gospel of Evolution has a wider significance than popular notions imply. The general idea, as to Evolution, that it is synonymous with Darwinism, is not accurate. The Darwinian teaching is only a part, though in one sense it is the most important part, of the Evolution truth. Evolution itself means, as we have seen, the unity of phenomena.
All things are, according to this new principle, one huge continuity. Whilst Darwinism shows that man is not distinct from the lower animals, and that all species of animals, and all species of plants are artificial groups gliding one into the other, just as in their gradual development they glided one out of the other, Evolution goes further than this and does not fare worse. For the evolutionist not only believes that which the works of Darwin have made an assured truth, but he believes that plants and animals have had a common parentage, that living matter has originated from the non-living, that there has been no break in the vast series of phenomena at any point.
Some of the general grounds for this belief have been given. Let us look rapidly at some of the more special. The principle of the conservation of energy already mentioned indirectly is, in a sense, the starting point of thought on this subject. Grove's essay on the "Correlation of the Physical Forces," published a few years since, was the first clear enunciation of the generalisation towards which so many observations had led. When he reminded men that chemical action, electricity, heat, sound, light, magnetism, and life were all convertible, one into the other, and thus convertible in definite numerical proportions, mathematically calculable, the keynote of the idea of Evolution had been struck.
Harsh as it may seem, an idea in any branch of knowledge has never attained a sure basis until it is expressible in terms of mathematics. There was a time when physics and chemistry were divorced from mathematics to a large extent. Now even the phenomena of electricity and the reactions one upon another of chemical bodies are expressed in algebraical formulae. This is the result of the increased precision of our knowledge. Following in the footsteps of physics and chemistry the biological sciences are becoming every day more mathematical. We have formulas to express the manner of the arrangement of leaves upon a stem, the manner of arrangement of the parts of a flower. One of these days every structural and functional fact in regard to every living thing will be related to some formula of mathematics more or less general. We shall not all become martinets or dryasdusts. There is a beauty in exactness. I sometimes think that the difference between the loveliness of our thinking and of our dreaming on natural phsenomena, as compared with that which the older thinkers and dreamers enjoyed, will be as the difference between the joy of a game of chess between skilled players or between those that know not even the moves. The child pushes the kings and queens and rooks and knights and bishops and pawns about at random, and laughs gaily. But the master' of the game, moving them according to definite rules, obtains a far higher enjoyment, and produces a combination that has its poetry.