
This is a misattribution. The Family and the Nation was written by William Cecil Whetham, not Arnold Gesell. Here's the full quote:
The American Birth Control League2 left no doubt about its plans. Its position, as expressed by Yale psychologist Arnold L. Gesell, was that "We need not wait for the perfection of the infant science of eugenics before proceeding upon a course of supervision and segregation which will prevent the horrible renewal of this defective protoplasm that is contaminating the stream of village life."[p.5] Gesell’s The Family and the Nation (1909), a thorough product of the new zeitgeist, advocated "eugenic violence" in dealing with inferiors. According to Gesell, "We must do as with the feebleminded, organize the extinction of the tribe."3