Tacitus, Germania. Chapter 4.
4. For my own part, I agree with those who think that the tribes of Germany are free from all taint of intermarriages with foregin nations, and that they appear as a distinct, unmixed race, like none but themselves. Hence, too, the same physical peculiarities throughout so vast a population. all have fierce blue eyes, red hair, huge frames, fit only for a sudden exertion. They are less able to bear laborious work. Heat and thirst they cannot in the least endure; to cold and hunger their climate and their soil inure them.
Tacitus, Germania - translated by Alfred Lord Church and William Jackson Brodribb.