36 ANIMAL PHYSIOLOGY. 
each other and the rest of the body; 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 be found in the development 
to an incomparably higher degree of his intellectual and moral 
nature, 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 a 
form more quickly available (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 minds 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, of 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. Re- 
currence 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, exem- 
plifies 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 
