Phases of spreading and intermixture have probably alternated with phases of settlement and specialization in the history of mankind. But up to a few hundred years ago it is probable that since the days of the Palæolithic Age at least mankind has on the whole been differentiating. The species has differentiated in that period into a very great number of varieties, many of which have reblended with others, which have spread and undergone further differentiation or become extinct. Wherever there has been a strongly marked local difference of conditions and a check upon intermixture, there one is almost obliged to assume a variety of mankind must have appeared. Of such local varieties there must have been a great multitude.
In one remote corner of the world, Tasmania, a little cut-off population of people remained in the early Palæolithic stage until the discovery of that island by the Dutch in 1642. They are now, unhappily, extinct. The last Tasmanian died in 1877. They may have been cut off from the rest of mankind for 15,000 or 20,000 or 25,000 years.
But among the numerous obstacles and interruptions to intermixture there have been certain main barriers, such as the Atlantic Ocean, the highlands, once higher, and the now vanished seas of central Asia and the like, which have cut off great groups of varieties from other great groups of varieties over long periods of time. These separated groups of varieties developed very early certain broad resemblances and differences. Most of the varieties of men in eastern Asia and America, but not all, have now this in common, that they have yellowish buff skins, straight black hair, and often high cheek-bones. Most of the native peoples of Africa south of the Sahara, but not all, have black or blackish skins, flat noses, thick lips, and frizzy hair. In north and western Europe a great number of peoples have fair hair, blue eyes, and ruddy complexions; and about the Mediterranean there is a prevalence of white-skinned peoples with dark eyes and black hair. The black hair of many of these dark whites is straight, but never so strong and waveless as the hair of the yellow peoples. It is straighter in the east than in the west. In southern India we find brownish and darker peoples with straight black hair, and these as we pass eastward give place to more distinctly yellow peoples.