One obstacle to a matured and detailed ethnological study of the Jew is the difficulty of becoming familiar with a people scattered over the two hemispheres. Though the race is one, the two great factors blood and climate have shown it to be anything but immutable, either in physique or in character. Compare, for instance, the two extremes--the Tatar-faced Karaïte of the Crimea with the Semitic features of Morocco, the blond lovelocks of Aden and the fiery ringlets of Germany with the greasy, black hair of Houndsditch. And as bodily form differs greatly, there is perhaps a still greater distinction in mental characteristics: we can hardly believe the peaceful and industrious Dutch Jew a brother of the fanatic and ferocious Hebrew who haunts the rugged Highlands of Safed in the Holy Land. Yet though these differences constitute almost a series of sub-races, there is one essentially great quality which cements and combines the whole house of Israel. The vigour, the vital force, and the mental capacity of other peoples are found to improve by intermixture; the more composite their character, the greater their strength and energy. But for generation after generation the Jews have preserved, in marriage at least, the purity of their blood. In countries where they form but a small percentage of the population the range of choice must necessarily be very limited, and from the very beginning of his history the Jew, like his half-brother the Arab, always married, or was expected to marry, his first cousin. A well-known traveller of the present day has proved that this can be done with impunity only by unmixed races of men, and that the larger the amount of mixture in blood the greater will be the amount of deformity in physique and morale to be expected from the offspring. Consanguineous marriages are dangerous in England, and far more dangerous, as De Hone has proved, in Massachusetts. Yet the kings of Persia intermarried with their sisters, and the Samaritan branch of the Jews is so closely connected that first cousins are almost sisters.