https://www.youtube.com/watch?v=wOR1wUqvJS4&feature=youtu.be

The new role of science -- procreation is becoming more and more voluntary, subject to man's control. In ancient times lack of understanding insured the appearance of children in the absence of all desire therefor.

  The new function of pleasure lures -- this introduces a new factor into racial survival; ancient man exposed undesired children to die; moderns refuse to bear them.

  The enhancement of parental instinct. Each generation now tends to eliminate from the reproductive stream of the race those individuals in whom parental instinct is insufficiently strong to insure the procreation of children, the prospective parents of the next generation.