Thus, this theory supposes that today migratory birds follow the path of a great racial movement that took place in a distant past, associated with advances and recessions of the ice. The actions of the birds themselves lend some support to this theory, as every bird student has noted the feverish impatience with which certain species push northward in spring, sometimes advancing so rapidly upon the heels of winter as to perish in great numbers when overtaken by late storms. It is probable, however, that at that season the reproductive impulse urges the birds on to their northern breeding grounds. SOUTHERN ANCESTRAL HOME THEORY The opposing theory is simpler in some respects and supposes that the ancestral home of all birds was in the Tropics and that as all bird life tends to overpopulation there was a constant effort to seek breeding grounds where competition would be less keen. Species that strove for more northern latitudes would be kept in check by the ice and forced to return southward with the recurrence of winter conditions. As the ice retreated, vast areas of virgin country became successively suitable for summer occupancy, but the winter habitat remained the home to which the birds returned after the nesting season. It is a fact that some species spend very little time on their breeding grounds; the orchard oriole (_Icterus spurius_), for example, spends only 2½ months in its summer home, arriving in southern Pennsylvania about the first week in May and leaving by the middle of July. Both theories assume that migration is an ingrained habit, but neither is supported by positive biological data. Both have been criticized also on geological grounds, and neither can be accepted without qualification. It is apparent, however, that whether the ancestral home of any species was at the northern or at the southern limits of its present range, or even in some intermediate region, the search for conditions favorable for breeding in summer and for feeding in winter has been a principal factor underlying the origin of migration. THEORY OF PHOTOPERIODISM