During the period from 1941-1945 the pattern of season of birth was largely European. From 1946-1950 the European pattern was modified by the beginning of a North American periodicity. From 1951-1955 the European pattern changed slightly, while the North American one became greatly enhanced. Between 1956-1961 the European periodicity was eliminated, the North American periodicity remained. Natality statistics show that with the exception of the New World the season of birth is largely meteorologically controlled. This does not appear to be the case in Puerto Rico or the northern continental USA. Such a change in pattern is probably due to the system of mass communications between Puerto Rico and the USA.