Physiology shows us anatomical differences between races.
There are as marked and real differences between the skull of a Hindoo and that of a Chinaman as between the skulls of an Englishman and a negro.
There is not as great a difference, perhaps, but it is as real and as constant.

Then the characters of races remain distinct, the same traits reappearing after many centuries exactly as at first.

We find the same difference of character between the Jews and Arabs, who are merely different families of the same Semitic race, as existed between their ancestors, Jacob and Esau, as described in the Book of Genesis.

Jacob and the Jews are prudent, loving trade, money-making, tenacious of their ideas, living in cities; Esau and the Arabs, careless, wild, hating cities, loving the desert.
A similar example of the maintaining of a moral type is found in the characteristic differences between the German and Kelts, two families of the same Indo-European race.

Take an Irishman and a German, working side by side on the Mississippi, and they present the same characteristic differences as the Germans and Kelts described by Tacitus and Caesar.