Hemisphere, though in some cases there were slight barriers tending to produce or maintain slight variations. The long longitudinal chain of the Sierra Nevada abounding in glaciers to a late date, and to a less extent that of the Rocky Mountains, brought about a partial isolation of the stocks in the great north-and-south migrations, maintaining previous differences and originating others, so that now we distinguish differences between what is called the Atlantic and what is called the Pacific group, while they are yet practically the same.[2] The tribes farthest advanced at the beginning of the isolation on this continent would not necessarily continue at the front of progress, for a change of conditions that might cripple such tribes might at the same time be beneficial to others previously inferior. For instance, as the heat gradually returned, the highly developed lowland tribes began to find themselves at a disadvantage, which grew with the intensity of the heat, while others, inured to harsher conditions, found warmth stimulating, and they began to develop germs received from the superior but now declining stocks. “The American Indians,” says Brinton, “cannot bear the heat of the tropics even as well as the European.” The heat, which at first seems to have been intense in the daytime, then caused a decline of the highest stocks, and a corresponding progression of lower stocks existing on, or migrating to, higher levels. The Yucatec tribes declined, while the Nahuatls, at higher altitudes, began to develop. The finest monuments of North American antiquity, for these reasons, are generally found on comparatively low levels and below a certain latitude, where conditions during the greatest cold were most favourable; conditions that may have continued fairly favourable down to within, say, a thousand years.