Let us now turn to the facts as we have them in the Kallikak family. The only offspring from Martin Kallikak Sr. and the nameless feeble-minded girl was a son who proved to be feeble-minded. He married a normal woman and had five feeble-minded children and two normal ones. This is in accordance with Mendelian expectation; that is to say, there should have been part normal and part defective, half and half, if there had been children enough to give the law of averages a chance to assert itself. The question, then, comes right there. Should Martin Jr. have been sterilized? We would thus have saved five feeble-minded individuals and their horrible progeny, but we would also have deprived society of two normal individuals; and, as the results show, these two normals married normal people and became the first of a series of generations of normal people. Taking this family as a whole, we have the following figures:-- There were 41 matings where both parents were feeble-minded. They had 222 feeble-minded children, with two others that were considered normal. These two are apparent exceptions to the law that two feeble-minded parents do not have anything but feeble-minded children. We may account for these two exceptions in one of several ways. Either there is a mistake in calling them normal, or a mistake in calling the parents feeble-minded; or else there was illegitimacy somewhere and these two children did not have the same father as the others of the family. Or we may turn to the Mendelian law and we discover that according to that law there might be in rare instances such a combination of circumstances that a normal child might be born from two parents that function as feeble-minded. For practical purposes it is, of course, pretty clear that it is safe to assume that two feeble-minded parents will never have anything but feeble-minded children. Again, we find that there were eight cases where the father was feeble-minded and the mother normal, and there were ten normal children and ten defective. There were twelve cases where the father was normal and the mother feeble-minded, with seven feeble-minded children and ten normal. Both of these are in accordance with Mendelian expectations. We further find that in the cases where one parent was feeble-minded and the other undetermined, the children were nearly all feeble-minded, from which we might infer that the probabilities are great that the unknown parent was also feeble-minded. We shall not go further into this matter in the present paper, but leave the detailed study of this family from the Mendelian standpoint for further consideration, when we take up the large amount of data which we have on three hundred other families. Enough is here given to show the possibility that the Mendelian law applies to human heredity. If it does, then the necessity follows of our understanding the exact mental condition of the ancestors of any person upon whom we may propose to practice sterilization.