But however unmistakably musical history may reveal an evolutionary process, it does not reveal that process as perfectly regular and uniform. That general tendency from a low toward a high state of organization, with increase in definiteness, coherence, and heterogeneity, which readers of Herbert Spencer expect in any evolutionary series, does characterize the growth of music as a whole; but within the large general process we also observe, as we do in many other cases of evolution of any degree of complexity, many momentary phases sharply marked off from one another, many separate and distinct periods, like the chapters in a book or the acts in a play. Each period, beginning tentatively, maturing slowly, and culminating in music which carries its characteristic effects to the highest possible pitch, is succeeded by another, presenting the same phases of growth, but seeking effects quite different. All the periods hang together in a large view; yet they are, after all, diverse in character, and therefore capable of being distinguished, and even dated. An analogy offered by certain well-known chemical processes may help to make comprehensible this periodic nature of musical evolution. Chemists have a term, “critical point,” by which they name a stage in the behavior of a substance, under some systematic treatment, at which it suddenly undergoes some striking change, some catastrophic transformation. Put, for example, a lump of ice in a crucible and apply an even heat by which its temperature is raised, say, one degree each minute. Here is a systematic treatment of the ice, a steady influence exerted upon it. Yet, curiously enough, this ice which is being so equably acted upon will not change its form in the equable, regular fashion we might expect. It will seem to undergo little or no change until, at a given moment, suddenly, it passes into water, a liquid wholly different in appearance from the original solid. It has reached a “critical point.” Continue the heating, and presently another critical point will be reached, at which, with equal suddenness, the liquid will be transformed into a vapor--steam. These catastrophes, in which the physical properties of the substance suddenly change, are conditioned, of course, by its chemical nature. They take place in the midst of a systematic treatment which we might expect to produce only gradual, inconspicuous effects, but which, as a matter of fact, produces a series of events as strikingly differentiated one from another as the acts of a drama.