GENERAL BIOLOGY. 37 



other and the rest of the hody ; minor differences in the hands 

 and feet, especially the greater freedom and power of apposition 

 of the great-toe. 



But the greatest distinction between man and even his closest 

 allies among the apes is to he found in the development to an 

 incomparably higher degree of his intellectual and moral na- 

 ture, corresponding to the differences in weight and structure 

 of the human brain, and associated with the use of spoken and 

 written language ; so that the experience of previous genera- 

 tions is not only registered in the organism (heredity), but in 

 the readily available form of books, etc. 



The greatest structural difference between the races of men 

 are referable to the cranium ; but, since they all interbreed 

 freely, they are to be.considered varieties of one species. 



THE LAW OF PERIODICITY OR RHYTHM IN NATURE. 



The term rhythm to most miads suggests music, poetry, or 

 dancing, in all of which it forms an essential part so simple, 

 pronounced, and uncomplicated as to be recognized by all with 

 ease. 



The regular division of music into bars, the recurrence of 

 chords of the same notes at certain intervals, oi forte and piano, 

 seem to be demanded by the very nature of the human mind. 

 The same applies to poetry. Even a child that can not under- 

 stand the language used, or an adult listening to recitations in 

 an unknown tongue, enjoys the flow and recurrences of the 

 sounds. Dancing has in all ages met a want in human organi- 

 zations, which is partly supplied in quieter moods by the regu- 

 larity of the steps in walking and similar simple movements. 



But as rhythm runs through all the movements of animals, 

 so is it also found in all literature and all art. Infinite variety 

 wearies the mind, hence the fatigue felt by the sight-seer. Ee- 

 eurrence permits of repose, and gratifies an established taste or 

 appetite. The mind delights in what it has once enjoyed, in 

 repetition within limits. Repetition with variety is manifestly 

 a condition of the growth and development of the mind. This 

 seems to apply equally to the body, for every single function of 

 each organism, however simple or complex it may be, exempli- 

 fi.es this law of periodicity. The heart's action is rhythmical 

 (beats) ; the blood flows in intermitting gushes from the central 

 pump ; the to-and-fro movements of respiration are so regular 



