I shall begin with some phenomena which are pretty generally recognised. Men when quite young, say under twenty, are attracted by much older women (say those of thirty-five and so on), whilst men of thirty-five are attracted by women much younger than themselves. So also, on the other hand, quite young girls (sweet seventeen) generally prefer much older men, but, later in life, may marry striplings. The whole subject deserves close attention and is both popular and easily noticed. In spite of the necessary limitation of this work to the consideration of a single law, it will make for exactness if I try to state the formula in a more definite fashion, without the deceptive element of simplicity. Even without being able to state in definite quantities the other factors and the co-operating laws, we may reach a satisfactory exactness by the use of a variable factor. The first formula was only an abstract general statement of what is common to all cases of maximum sexual attraction so far as the sexual relation is governed by the law. I must now try to find an expression for the strength of the sexual affinity in any conceivable case, an expression which on account of its general form, can be used to describe the relationship between any two living beings, even if these belong to different species or to the same sex. If { α M { β W χ { and γ { { α´ W { β´ M (where α, α´, β, and β´ are each greater than 0 and less than unity) define the sexual constitutions of any two living beings between which there is an attraction, then the strength of the attraction may be expressed thus: K A = -----_f^t_ α - β where _f^t_ is an empirical or analytical function of the period during which it is possible for the individuals to act upon one another, what may be called the “reaction-time”; whilst K is the variable factor in which we place all the known and unknown laws of sexual affinity, and which also varies with the degree of specific, racial and family relationship, and with the health and absence of deformity in the two individuals, and which, finally, will become smaller as the actual spacial distance between the two is greater, and which can be determined in any individual case. When in this formula α = β A must be infinity; this is the extreme case; it is sexual attraction as an elemental force, as it has been described with a weird mastercraft by Lynkeus in the novel “Im Postwagen.” Such sexual attraction is as much a natural law as the downward growth of a rootlet towards the earth, or the migration of bacteria to the oxygen at the edge of a microscopic cover-glass. But it takes some time to grow accustomed to such a view. I shall refer to this point again. If α - β has its maximum value, which is when it equals unity, then A = K. _f^t_.