It is in a similar way that, in the history of music, the tonal material used, under the systematic treatment of man’s æsthetic faculty, has been constrained by its nature to undergo sudden changes, to recrystallize in novel ways, to take on unwonted aspects which initiate new periods. When the possibilities of one sort of tone-combination are nearly or quite exhausted, the keener minds of a generation, led by groping but unerring instinct, grasp an unused principle of organization, latent in the material, and inaugurate a new style. This in turn runs its course, develops its resources, reaches its perfection, and is succeeded by another, which, after due time, is also superseded. All these periods are but moments in one vast evolution, successive blossomings from the one root of human feeling expressible in music; yet each has its individual qualities, its peculiar style, its special masters. It is possible both to trace certain general tendencies through them all, and to define other special qualities in which each is peculiar; and it will be worth while, before passing on to our proposed study of the particular period of Beethoven, to describe thus in general terms the salient features of the evolution as a whole, and to characterize, however briefly, the individual periods we can discriminate in it. In the most general point of view, an evolution, of whatever sort, is a progress from what Spencer calls “indefinite, incoherent, homogeneity,” to what, consistently if rather overwhelmingly, he calls “definite, coherent, heterogeneity.” All low forms of life, that is to say, are so homogeneous in constitution as to be comparatively indefinite and incoherent; their parts, being all very much alike, cannot be built up into definite, strongly cohesive structures. A jelly fish, made up of thousands of but slightly differentiated cells, and without legs, arms, head, or any viscera worth mentioning except stomach, is doubtless a useful animal, but not one of pronounced individuality or solidarity. A savage tribe, consisting of many human beings almost indistinguishable from one another as regards character, strength, accomplishments, or powers of leadership, is a similar phenomenon in a different field, a sort of social jelly fish.