Apaches and Navajos are usually said to have _come down_ from their kin in the North, but it is equally possible that they _remained behind_ in the high mountains while their kin pushed on.[398] The table-lands of Mexico, being high and temperate, formed a final refuge for many tribes, some of whom had profited by contact with the centre of development, and these roamed the plateau, one branch finally settling around the lake of Mexico, and there planting again the seeds of the lowland culture. Many tribes were early crowded into the California coast region, because the lowland climate there remained comparatively mild, and the supply of fish, seals, etc. was so great that they were not compelled to till the soil for subsistence (if indeed they were possessed of sufficient knowledge, or if the land were in condition to produce), as was the case farther south, where the population was denser and natural supplies insufficient. But the region was so inhospitable that only fragments of these tribes survived. They did not multiply. [Illustration: PROBABLE ASPECT OF ALASKA SUMMER LANDSCAPE SOME 600 YEARS AGO Harriman Alaska Expedition, 1899. Photographed by the author ] The reason the Eastern continents produced many and diverse peoples is that the glacial period temperate zone, or warm zone, extended through many degrees of _longitude_, offering extensive areas of settlement to the races in that hemisphere, where they remained more or less isolated and independent, to advance in their own way and along their own lines; that is, on the Eastern continents there was ample _latitudinal_ land space, while on the Western there was a very limited latitudinal land space that retained a salubrious climate. This was the cause of North American race homogeneity.