The Lord’s manifestations correlate with the evolution of the serving mood of the jiva

We can notice the different stages of animal life from the invertebrates to the fully-grown human beings. These stages have been classified by the Indian sages of a scientific outlook in ten orders, viz, (1) the invertebrate, (2) testaceous or shelly, (3) vertebrate, (4) erectly vertebrate (as in the combined form of man and beast), (5) mannikin, (6) barbaric, (7) civilised, (8) wise, (9) ultra-wise and (10) destructive. These are the historical stages of jivas [souls]. According to the gradation of these stages as indications of evolution of the serving mood of the jiva soul, there are manifested the ten Incarnations of God, viz., Matsya (fish), Kurma (Turtle), Varaha (Boar), Nrishmha (Man-Lion), Vamana (Dwarf), Parasu-rama, Rama, Krishna, Buddha and Kalki, as worshippable Deities with eternal transcendental Names, Forms, Attributes and Sports.

Conversation between Bhaktisiddhanta Sarasvati Thakur and Albert E. Suthers, The Harmonist (Sree Sajjanatoshani), January 1929 issue.