But since the selection rule that musical evolution seems to have applied is that the melodic mould should be aesthetically satisfying, the number of ragas is about three hundred of which only about a hundred are common. Several attempts have been made to group the ragas in relation to primaries and derivatives. Rhythmic Patterns Singing and playing in Indian music can be slow, medium or fast and this determines the tempo. The north Indian music has explored the possibilities of the slow tempo perhaps more fully than the south Indian and it accelerates in minimal gradations. In south Indian music the medium tempo is precisely the double of the slow, and the fast precisely the double of the medium.
Indian rhythms are apt to be more difficult for foreigners to grasp.
This is because a single cycle of rhythm, or bar, can be built out of units of different duration. The total duration for the cycle can be divided in various ways. When the cycles are repeated in the continuous singing or playing, the pattern of accenting may vary even if the total duration of two rhythm-schemes is the same, and the cycle itself may be long. Equipped with melody and rhythm, music has undergone a magnificent evolution in India. In its sweep it has united earth and sky.