Srila Prabhupada’s Letter to Dr. W. H. Wolf-Rottkay, June 18, 1975:
Biological evolution is taking place on account of the soul’s desire. It takes place because of infection. If a man has smallpox virus, then he is subjected to smallpox. It is not that everyone is infected and everyone is attacked with smallpox disease. Only the person who is infected, he is attacked.
In this connection, Krishna says in the Bhagavad-gita, purusah prakrti-stho hi bhunkte prakrti-jan gunan karanam guna sango ’sya