Havelock Ellis,
Studies in the Psychology of Sex…
“ It is not difficult to see why this normal association of sexual emotion with suckling should have come about. It is essential for the preservation of the lives of young mammals that the mothers should have an adequate motive in pleasurable sensation for enduring the trouble of suckling. The most obvious method for obtaining the necessary degree of pleasurable sensation lay in utilizing the reservoir of sexual emotion, with which channels of communication might already be said to be open through the action of the sexual organs on the breasts during pregnancy. ”
