It is impossible within the limits of the present essay to trace the embodiment of all these laws of Nature in the different branches of Art; and, besides, science as yet has only brought us to the threshold of Nature's storehouse, and therefore we are still ignorant of the vast variety of her action, and the fulness of meaning of the principles according to which she acts. It may be sufficient, however, to take one of these principles, the Law of Continuity, which has been so remarkably expanded by the later researches of science; and with it the complementary law, the Law of Variety. To take the latter first; throughout all external Nature there is no sameness, no monotony; no two successive events are exactly the same, no two objects have exactly the same qualities, whether they be two undulations of light or whether they be two offspring of the same parents; this principle is of immense importance in science, as in the theory of the Variation of Species, for instance. But the principle of continuity is even more so; it tells us that through all this variety runs a perfect oneness of nature, linking all things together with the ties of a common brotherhood. The physical phenomena of the external world, sound, light, heat, electricity, have a common origin; they may really be looked upon as what the botanist or zoologist would call varieties of the same species; while in botany and zoology again this blending of species is unfailingly present. Moss and fern, fish and reptile, even animal and vegetable; who can draw the line between them? a kindred nature seems to envelop every form of life.
Now let us turn to Art; and take that of the simplest kind, the expression of beauty in form merely. Both these laws must be present together. The straight line is continuous, but it is certainly monotonous, and therefore perhaps the least beautiful of all lines. The circle is better, there is a change of direction, but a want of variety about that change; the line keeps on bending always at the same rate; an ellipse or oval is more beautiful, and curves, such as the parabola or hyperbola, which may suggest infinite possibilities of change, best of all. On the other hand, a broken line, or a curve which suddenly changes its direction, are not beautiful; and it is to be remarked that such curves do absolutely not occur in Nature; though many lines do approach the case so nearly as to suggest discontinuity, and with it to suggest a painful sense of incompleteness. With colouring, again, the importance of these principles should never be overlooked; a monotonous shade of colour is not known in Nature and ought not to be in Art; on the other hand, discontinuity of colouring is equally faulty; every colour modifies the adjacent colours by throwing over them a tinge of its complementary colour, and that not suddenly but by infinitely delicate gradations, and to disregard this is as wanting to Art as it is untrue to Nature. If we turn to art in sound, though the material is different, we may trace these same principles (amongst others) with as much certainty as before. There is no melody, for instance, in the repetition of the same sound without variety; more beauty is found in a chromatic scale proceeding by semitones; but here there is a monotony in the intervals, and the diatonic scale is more pleasing. Introduce varieties in time (subject, of course, to other laws, such as that of order, which may here come in) and the effect is better, and, as a general rule, the more complete the variety introduced, in the tones of the notes, in the intervals of sound between them, in the intervals of time between them, the more complete is the impression; provided always this variety be introduced subject to an internal continuity. The continuity of two sounds is perhaps greatest, that is, there is the least real change produced in the movement of the conveying air or the receiving ear, not when they are nearest together on the keyboard of a piano, or the page of music, but when they are related by some simple interval, such as an octave or a fifth or fourth. This is sufficiently shewn by the difficulty sometimes found in making a large pipe sound its proper bass note; it will glide off into the octave above on the smallest provocation. The next approach to continuity is obtained in the delicate gradation from one note to another through the intermediate sounds, as when a violinist slides his finger along the string from one position to another. According with this, we find that the ruder forms of melody, as the jodeling of the Swiss, incline more to the use of harmonic intervals, thirds, fifths, and octaves; while modern music has shewn a great tendency towards chromatic modulation. But in every beautiful air, that perfect continuity of sound is always one of the most remarkable features. It is worth the trouble to take some well-known melody which commends itself for its beauty, and to examine it with respect to these two principles only. The continuity will be shewn not only in the absence of abrupt successions of notes, but also in the absence of abrupt changes of style or time, and in the continuance of the same theme or idea throughout the whole. On the other hand, it will be found that the same notes or intervals never recur monotonously, the theme is never treated twice in exactly the same way; if the air is ascending in one part, it descends again in another, if the time is quickened in one part, it is lengthened out in another, and so on. All the, pleasure of an air with variations is due to this principle of continuance under variety of form.
So we might take the principle of power associated with perfect moderation, or of complete individuality in every part associated with sympathetic harmony throughout, or whatever other principle Nature affords, and so trace their importance in every branch of Art. But, for our purpose here, it is sufficient to allow that, in respect of all these ideas, all branches of Art, whatever be their origin, are one; and thus the question before us will be very much simplified, and, by allowing this unity of Art, we shall be able to avoid the mistake of giving undue preference to any branch to which we may be naturally inclined, and shall be ready, if we perceive any one particular advantage to spring from one branch of Art, to allow the possibility of its accruing too from other branches.