Genera, at first constituted of but few species, have for a time gone ongrowing more multiform, and then have declined in the number of their subdivisions: leaving at last but one or two, or none at all. During longer epochs wholeorders have thus arisen, culminated, and dwindled away. And even those widerdivisions containing many orders have similarly undergone a gradual rise,a high tide, and a long-continued ebb. The stalked Crinoidea, for example,which during the carboniferous epoch became abundant, have almost disappeared: only a single species being extant. Once a large family, the Brachiopodahave now become rare. The shelled Cephalopods, at one time dominant amongthe inhabitants of the ocean, both in number of forms and of individuals,are in our day nearly extinct. And after an "age of reptiles" hascome an age in which reptiles have been in great measure supplanted by mammals.

Thus Life on the Earth has not progressed uniformly, but in immense undulations. §86. It is not manifest that changes of consciousness are in anysense rhythmical. Yet here, too, analysis proves both that the mental stateexisting at any moment is not uniform, but is decomposable into rapid oscillations,and also that mental states pass through longer intervals of increasing anddecreasing intensity.

Though while attending to any single sensation, or any group of relatedsensations constituting the consciousness of an object, we seem to remainin a persistent and homogeneous condition of mind, self-examination showsthat this apparently unbroken mental state is traversed by many minor states,in which various other sensations and preceptions are rapidly presented anddisappear. As thinking consists in the establishment of relations, it followsthat continuance of it in any one state to the entire exclusion of otherstates, would be a cessation of thought, that is, of consciousness. So thatany seemingly uniform feeling, say of pressure, really consists of portionsof that feeling perpetually recurring after momentary intrusions of otherfeelings and ideas -- quick thoughts concerning the place where it is felt,the external object producing it, its consequences, etc. Much more conspicuousrhythms, having longer waves, are seen during the outflow of emotion intodancing, poetry and music. The current of mental energy expended in one ofthese modes of bodily action, is not continuous but falls into successivepulses. The measure of a dance is produced by the alternation of strong muscularcontractions with weaker ones; and, save in measures of the simplest order,such as are found among barbarians and children, this alternation is compoundedwith longer rises and falls in the degree of muscular excitement. Poetryis a form of speech in which the emphasis is regularly recurrent, that is,inwhich the muscular effort of pronunciation has definite periods of greaterand less intensity: periods that are complicated with others answering tothe successive verses. Music more variously exemplifies the law. There arethe recurring bars, in each of which there is a primary and a secondary beat.