A young man of good ancestry broke the moral law about one hundred and forty years ago and became the father of an illegitimate son by a feeble-minded mother. Of 480 descendants of this son, there have been 46 normal, many immoral, many alcoholic and 143 feeble-minded. The same man who back in the revolutionary days made a moral mistake which led to such awful consequences, later married a woman of good family and became the progenitor of a second line of 496 descendants of whom 494 have been normal mentally, while two were affected by alliance with another family; and all have been first-class citizens, many of them prominent in business, professions, etc. Even making due allowance for the depressing influence of the environment in which most of the down-and-out descendants in the degenerate line lived, the comparison between the normal and the abnormal lines from the same ancestor gives the most convincing eugenic evidence that has been discovered in the human race. Doubtless it will long be used as a basis for attempted biological control of the propagation of the unfit. Many similar cases of hereditary degeneracy are recorded in books on eugenics. Such a eugenic record as that of this Kallikak family should be reviewed in every high school and college in connection with the topic "heredity" in a course of biology, for it will teach two important lessons: (1) The biological principle that defects, both physical and mental, are highly heritable, even for many generations; and (2) the ethical responsibility for the sex actions of the individual who may start a long train of human disaster that may visit the children unto even later than the third and fourth generations. The first lesson is a purely biological one which suggests the eugenic argument that defective humans, like undesirable animals and plants, should not take part in the perpetuation of the species. The second lesson is not biological but ethical, suggesting individual responsibility for conduct which may disastrously affect other individuals' lives. It seems to me that so far as general education is concerned, the ethical lesson is the more impressive and more likely to lead to voluntary eugenic practice by individuals. It is my observation that even many intelligent people are not seriously impressed by the biological evidences for eugenics considered as a general problem, but their reaction is one of interest when one begins to present the question of ethical responsibility for the transmission of physical and mental defects to future generations. Such considerations have led me to the view, already suggested, that eugenic studies in courses of biology have their greatest practical value in their ethical implications, which, of course, by influencing individual responsibility for reproduction may lead to the desirable biological improvement of the human race. Teachers of biology should present, as an economic problem,