In the value of the factor “_t_,” the time of reaction, a final analogy of sexual attraction with chemical processes may be found, if we are willing to trace the comparison without laying too much stress upon it. Consider the formula for the rapidity of the reaction, the different degrees of rapidity with which a sexual attraction between two individuals is established, and reflect how the value of “A” varies with the value of “_t_.” However, what Kant termed mathematical vanity must not tempt us to read into our equations complicated and difficult processes, the validity of which is uncertain. All that can be implied is simple enough; sensual desire increases with the time during which two individuals are in propinquity; if they were shut up together, it would develop if there were no repulsion, or practically no repulsion between them, in the fashion of some slow chemical process which takes much time before its result is visible. Such a case is the confidence with which it is said of a marriage arranged without love, “Love will come later; time will bring it.” It is plain that too much stress must not be laid on the analogy between sexual affinity and purely chemical processes. None the less, I thought it illuminating to make the comparison. It is not yet quite clear if the sexual attraction is to be ranked with the “tropisms,” and the matter cannot be settled without going beyond mere sexuality to discuss the general problem of erotics. The phenomena of love require a different treatment, and I shall return to them in the second part of this book. None the less, there are analogies that cannot be denied when human attractions and chemotropism are compared. I may refer as an instance to the relation between Edward and Ottilie in Goethe’s “Wahlverwandtschaften.” Mention of Goethe’s romance leads naturally to a discussion of the marriage problem, and I may here give a few of the practical inferences which would seem to follow from the theoretical considerations of this chapter. It is clear that a natural law, not dissimilar to other natural laws, exists with regard to sexual attraction; this law shows that, whilst innumerable gradations of sexuality exist, there always may be found pairs of beings the members of which are almost perfectly adapted to one another. So far, marriage has its justification, and, from the standpoint of biology, free love is condemned. Monogamy, however, is a more difficult problem, the solution of which involves other considerations, such as periodicity, to which I shall refer later, and the change of the sexual taste with advancing years.