Sri Aurobindo
Letters of Sri Aurobindo
7. The Purpose of Avatarhood
Avatarhood would have little meaning if it were not connected 
with the evolution. The Hindu procession of the ten Avatars is itself, as it were, a parable of evolution. First the Fish Avatar, then the amphibious animal between land and water, then the land animal, then the Man-Lion Avatar, bridging man and animal, then man as dwarf, small and undeveloped and physical but containing in himself the godhead and taking possession of existence, then the rajasic, sattwic, nirguna Avatars, leading the human development from the vital rajasic to the sattwic mental man and again the overmental superman. Krishna, Buddha and Kalki depict the last three stages, the stages of the spiritual development – Krishna opens the possibility of overmind, Buddha tries to shoot beyond to the supreme liberation but that liberation is still negative, not returning upon earth to complete positively the evolution; Kalki is to correct this by bringing the Kingdom of the Divine upon earth, destroying the opposing Asura forces. The progression is striking and unmistakable.
As for the lives in between the Avatar lives, it must be remembered that Krishna speaks of many lives in the past, not only a few supreme ones, and secondly that while he speaks of himself as the Divine, in one passage he describes himself as a Vibhuti, vṛṣṇīnāṃ vāsudevaḥ. We may therefore fairly assume that in many lives he manifested as the Vibhuti veiling the fuller Divine Consciousness. If we admit that the object of Avatarhood is to lead the evolution, this is quite reasonable, the Divine appearing as Avatar in the great transitional stages and as Vibhutis to aid the lesser transitions.