In the narrow Isthmian region there are tribes of Indians that resist manfully the inroads of civilization. Perhaps the best known of these are the San Blas Indians who inhabit the mountain fastnesses east of the Canal Zone. In northern Costa Rica the Guatuso and Talamanca tribes still maintain to a considerable degree their old native character.
Minor physical differences in stature, head form, and facial expression mark off pretty clearly the tribes of this area from each other. The stature is lowest among the Mayas and Mazatecs, the average being about 5 feet 1 inch, while among the Tarascans, Tlaxcalas, and Zapotecs, it averages about 5 feet 3 inches. The other tribes of Central America and of central Mexico fall between these extremes. In northern Mexico the stature increases considerably, average measurements for the Yaqui being in excess of 5 feet 6 inches. To make up for their lack of height the southern Indians are sturdy and heavy muscled, with deep chests. Their hair is usually black and straight, but occasionally wavy. Light beards and mustaches are sometimes worn, especially by the Mayas. The eyes are so dark brown as to appear black to the casual observer. They are set rather wide apart and while usually horizontal they seem, in some instances, to have a slight Mongoloid tilt. Noses vary greatly but are often finely aquiline. The cephalic index (obtained by dividing the breadth of the head by its length and multiplying the result by 100) is rather high. The Mayas are strongly round-headed with an index of 85.0 while their linguistic relatives, the Tzendals, have a medium index of 76.8. The other tribes of southern Mexico fall between these extremes. No long-headed peoples are found in this area although in northern Mexico some tribes approach the long-headed type.
[Illustration: Map of Mexico and Central America showing the Principal Archæological Sites with a Detail Insert of the Valley of Mexico. High-resolution Map]
In 1910 an actual stratification of human products was found in the environs of Mexico City in which three principal culture horizons could be readily discerned. A collection made at the time, illustrating the objects characteristic of the three strata, is on exhibition in the American Museum of Natural History. In parts this stratification verified theories of culture succession already held by students working in this field. Since that time careful research in several localities has been carried on and many authentic specimens from the three layers have been brought together.