While Mr. Jefferson, when he wrote into the Declaration of Independence his belief in the self-evidence of the truth that all men are created equal, may have been thinking of legal rights merely, he was expressing an opinion common among philosophers of his time. J. J. Rousseau it was who made the idea popular, and it met with widespread acceptance for many years. It is not surprising, therefore, that the phrase has long been a favorite with the demagogue and the utopian. Even now the doctrine is by no means dead. The American educational system is based largely on this dogma, and much of the political system seems to be grounded on it. It can be seen in the tenets of labor unions, in the practice of many philanthropies--traces may be found almost anywhere one turns, in fact. Common enough as applied to mental qualities, the theory of human equality is even more widely held of "moral" qualities. Men are considered to be equally responsible for their conduct, and failure to conform to the accepted code in this respect brings punishment. It is sometimes conceded that men have had differing opportunities to learn the principles of morality; but given equal opportunities, it is almost universally held that failure to follow the principles indicates not inability but unwillingness. In short, public opinion rarely admits that men may differ in their inherent capacity to act morally. In view of its almost universal and unquestioned, although half unconscious, acceptance as part of the structure of society, it becomes of the utmost importance that this doctrine of human equality should be examined by scientific methods. Fortunately this can be done with ease. Methods of mental and physical measurement that have been evolved during the last few decades offer results that admit of no refutation, and they can be applied in hundreds of different places. [Illustration: DISTRIBUTION OF 10-YEAR-OLD SCHOOL CHILDREN FIG. 8.--The graph shows that 10-year-old children in Connecticut (1903) are to be found in every grade, from the first to the eighth. The greatest number is in the fourth grade, and the number who are advanced is just about the same as the number who are retarded.] It will not be worth while to spend any time demonstrating that all individuals differ, at birth and during their subsequent life, physically. The fact is patent to all. It carries with it as a necessary corollary mental differences, since the brain is part of the body; nevertheless, we shall demonstrate these mental differences independently.