Eugenics seeks for quantitative results. It is not contented with such vague words as ‘much’ or ‘little,’ but endeavours to determine ‘how much’ or ‘how little’ in precise and trustworthy figures. A simple example will show the importance of this. Let us suppose a class of persons, called _A_, who are afflicted with some form and some specified degree of degeneracy, as inferred from personal observations, and from family history, and let class _B_ consist of the offspring of _A_. We already know only too well that when the grade of _A_ is very low, that of the average _B_ will be below par and mischievous to the community, but how mischievous will it probably be? This question is of a familiar kind, easily to be answered when a sufficiency of facts have been collected. But a second question arises. What will be the trustworthiness of the forecast derived from averages when it is applied to individuals? This is a kind of question that is not familiar, and rarely taken into account, although it too could be answered easily as follows. The average mischief done by each _B_ individual to the community may for brevity be called _M_: the mischiefs done by the several individuals differ more or less from _M_ by amounts whose average may be called _D_. In other words _D_ is the average amount of the individual deviations from _M_. _D_ thus becomes the measure of untrustworthiness. The smaller _D_ is, the more precise the forecast, and the stronger the justification for taking such drastic measures against the propagation of class _B_ as would be consonant to the feelings if the forecast were known to be infallible. On the other hand, a large _D_ signifies a corresponding degree of uncertainty, and a risk that might be faced without reproach through a sentiment akin to that expressed in the maxim ‘It is better that many guilty should escape than that one innocent person should suffer.’ But that is not the sentiment by which natural selection is guided, and it is dangerous to yield far to it. There can be no doubt that a thorough investigation of the kind described, even if confined to a single grade and to a single form of degeneracy, would be a serious undertaking. Masses of trustworthy material must be collected, usually with great difficulty, and be afterwards treated with skill and labour by methods that few at present are competent to employ. An extended investigation into the good or evil done to the State by the offspring of many different classes of persons, some of civic value, others the reverse, implies a huge volume of work sufficient to occupy Eugenics laboratories for an indefinite time. OBJECT LESSONS IN THE METHODS OF BIOMETRY.