Climatic influences are persistent, often obdurate in their control. Arid regions permit agriculture and sedentary life only through irrigation. The economic prosperity of Egypt to-day depends as completely upon the distribution of the Nile waters as in the days of the Pharaohs. The mantle of the ancient Egyptian priest has fallen upon the modern British engineer. Arctic explorers have succeeded only by imitating the life of the Eskimos, adopting their clothes, food, fuel, dwellings, and mode of travel. Intense cold has checked both native and Russian development over that major portion of Siberia lying north of the mean annual isotherm of degree C. (32 degrees F.); and it has had a like effect in the corresponding part of Canada. (Compare maps pages 8 and 9.) It allows these sub-arctic lands scant resources and a population of less than two to the square mile. Even with the intrusion of white colonial peoples, it perpetuates the savage economy of the native hunting tribes, and makes the fur trader their modern exploiter, whether he be the Cossack tribute-gatherer of the lower Lena River, or the factor of the Hudson Bay Company. The assimilation tends to be ethnic as well as economic, because the severity of the climate excludes the white woman. The debilitating effects of heat and humidity, aided by tropical diseases, soon reduce intruding peoples to the dead level of economic inefficiency characteristic of the native races. These, as the fittest, survive and tend to absorb the new-comers, pointing to hybridization as the simplest solution of the problem of tropical colonization. [Sidenote: The relation of geography to history.]