It is a strange fact that although almost all professional paleontologists are agreed that existing data oppose the theory of continental drift, those who support it contend that their case is strengthened by these same data. If, in the geologic history of the earth, there was any such thing as continental drift, it appears from the evidence available that it was before the Cretaceous period, estimated to have been about 70,000,000 years ago. Birds had then evolved but those known from fossil remains were of extremely primitive types such as Hesperornis and Ichthyornis. There is no evidence of the existence in that period of any birds that were even closely related to any of those now living. Accordingly, it is difficult to believe that the migratory patterns of existing species have been determined by events that, if they did take place, were at least 70,000,000 or more years ago. _When Birds Migrate_ It is known that at any given point many species leave in the fall and return in the spring. Since banding has had such wide application as a method of study, it is known also that in some species one of the parent birds (rarely both) frequently returns and nests in the tree, bush, or box that held its nest in the previous season. One ordinarily thinks of the world of birds as quiescent during two periods each year, at nesting time, and in winter. For individuals this is obviously the case, but when the entire avifauna of the continent is considered it is found that there are at almost all periods some latitudinal movements. =Movements of species and groups= Some species begin their fall migrations early in July and in some parts of the country distinct southward movements can be detected from then until the beginning or middle of winter. For example, many shore birds start south in the early part of July, while the goshawks, snowy owls, redpolls, Bohemian waxwings, and many others do not leave the North until forced to do so by the advent of severe winter weather, or by lack of the customary food. Thus an observer in the northern part of the United States may record an almost unbroken southward procession of birds from midsummer to winter, and note some of the returning migrants as early as the middle of February. While on their way north, purple martins have been known to arrive in Florida late in January and, among late arrivals, the northern movement may continue into the first week of June. In some species the migration is so prolonged that the first arrivals in the southern part of the breeding range will have performed their parental duties while others of that species are still on their way north.