186 ETHICAL ASPECTS OF EVOLUTION 



rhythm we mean a series of equal beats or intervals. The 

 length of each beat is immaterial within certain limits, which 

 will vary with the nervous susceptibility of the individual. 

 They must not be so short as to fuse into a continuum ; that 

 is to say, they must be distinctly apprehended as separate ; 

 and they must not be of more than a certain length. If they 

 exceed that, the immediate sense of equality will be lost, and 

 they will themselves require measurement ; they will cease to 

 be rhythmical. Within these limits it is immaterial whether 

 intervals are seconds or pulse-beats, or the breaking of 

 waves on a beach, inches or centimeters, or the marks 

 made on the sands by a retreating tide. All concomitant 

 sensations are equally devoid of importance : it is immaterial 

 whether we measure with a tape or an iron rod. Rhythm 

 is number in a concrete form ; that is to say, combined with 

 sensation. Any kind of sensation will serve as its vehicle ; 

 it may be heard, or seen, or felt. An essential requirement 

 in rhythm, in order that it can be used for measurement, 

 is that the beats should be equal subdivisions of some 

 objective standard that is equal for all men such as the 

 length of a day at the equinox, or of a bar of metal under 

 certain conditions of temperature or pressure. Series of 

 equal arithmetical units of universal validity, such as can 

 be used in measurement, are given us in time and space 

 only. 



Measurements are of two kinds, direct and indirect. They 

 are direct when both the object measured and the rhythm 

 which measures it are in the same category ; as when we 

 measure a mile by inches, or an hour by seconds. They 

 are indirect when a correspondence has been established 

 between two series in different categories, and one is taken 

 to measure the other. The measurement of time by 

 a clock may be either direct or indirect. When a minute 

 is ascertained by counting the seconds, it is direct ; when 

 the same duration is measured by the space which the hands 

 have covered, it is indirect. It is hardly correct to say 

 that we measure time by space ; but to measure space, 

 on the other hand, we employ time. The length of the 



