THE RHYTHM OF MOTION. 273 



molluscs, the Brachiopoda have now become rare. The 

 shelled Cephalopods, at one time dominant among the in- 

 habitants of the ocean, both in number of forms and of 

 individuals, are in our day nearly extinct. And after an 

 " age of rej)tiles," there has come an age in which reptiles 

 have been in great measure supplanted by mammals. 

 Whether these vast rises and falls of different kinds of life 

 ever undergo anything approaching to repetitions, (which 

 they may possibly do in correspondence with those vast 

 cycles of elevation and subsidence that produce continents 

 and oceans,) it is sufficiently clear that Life on the Earth 

 has not progressed uniformly, but in immense undulations. 



§ 86. It is not manifest that the changes of conscious- 

 ness are in any sense rhythmical. Yet here, too, analysis 

 proves both that the mental state existing at any moment is 

 not uniform, but is decomposable into rapid oscillations; 

 and also that mental states pass through longer intervals of 

 increasing and decreasing intensity. 



Though while attending to any single sensation, or any 

 group of related sensations constituting the consciousness of 

 an object, we seem to remain for the time in a persistent and 

 homogeneous condition of mind, a careful self-examination 

 shows that this apparently unbroken mental state is in truth 

 traversed by a number of minor states, in which various 

 other sensations and perceptions are rapidly presented and 

 disappear. From the admitted fact that thinking consists 

 in the establishment of relations, it is a necessary corollary 

 that the maintenance of consciousness in any one state to the 

 entire exclusion of other states, would be a cessation of 

 thought, that is, of consciousness. So that any seemingly 

 continuous feeling, say of pressure, really consists of por- 

 tions of that feeling perpetually recurring after the mo- 

 mentary intrusion of other feelings and ideas — quick 

 thoughts concerning the place where it is felt, the exter- 

 nal object producing it, its consequences, and other things 



