For the source of any characteristic so widespread and uniform as this adaptation to environment we must go back to the very beginning of the human race. ↗
Geologists are rapidly becoming convinced that the mammals spread from their central Asian point of origin largely because of great variations in climate. ↗
Nevertheless most of the evergreen forests of the north must always remain the home of wild animals and trappers, a backward region in which it is easy for a great fur company to maintain a practical monopoly. ↗
The human organism inherits so delicate an adjustment to climate that, in spite of man's boasted ability to live anywhere, the strain of the frozen North eliminates the more nervous and active types of mind. ↗
The Indians could not undertake any widespread cultivation of the plains not only because they lacked iron tools but also because they had no draft animals. ↗