IN the account of the discussion in Section I (Physiology) of the British Association on "Health and the Community", which appeared in NATURE of September 18, p. 493, it is stated on p. 494 that the safest rate of reproduction for both mother and offspring is the modal rate. The author of the article has asked us to point out that this statement holds for any size of family, but the example of modal rate actually given was for a family of seven. This does not bear the implication that seven is the ideal number per family. Actually the lowest mortality was found in families of three, produced at the modal rate. The author has also sent the following supplementary note. "One point in the discussion of immense importance for the future not only of the race but also threatening civilization itself, was not sufficiently stressed in the article, namely, the differential birth rate. It follows from the facts that the modal rate is, on the whole, observed only in the lowest paid members of the community, and that they start reproduction early (less than twenty years of age), that the number of the population below the poverty line, that is, those being maintained by the tax-payers, will steadily increase. In 1930, round about 10 per cent of the families had more than 40 per cent of the pregnancies, 45 per cent of the still-births, 53 per cent of the infant plus child mortalities and 63 per cent of the miscarriages. In 1950, the 10 per cent will have become at least 30 per cent. Nothing short of massacre can alter that now. Births cannot be retrospective. If nothing is done and the process goes on, in 1970, the submerged tenth will be multiplied by seven. This means that, apart altogether from the increasing load of old people, some 30 per cent of the population will have to face the problem of supporting the rest of the community."