The head among Old Americans is in many cases notable for its good development, particularly in males. Among 12 groups of male immigrants[201] measured at Ellis Island under Dr. Hrdlicka's direction in recent years, not one group quite equals in this respect the Americans, the nearest approach being noted in the Irish, Bohemians, English, Poles, and North Italians. The type of head, however, differs among the Americans very widely, as is the case with most civilized races at the present day. Head form is most conveniently expressed by means of the cephalic index, that is, the ratio of breadth to length. Anthropologists generally speak of any one with an index of 75 (or where the breadth is 75% of the length) and below this as dolichocephalic, or long-headed; from 75 to 80 is the class of the mesocephalic, intermediates; while above 80 is that of the subbrachycephalic and brachycephalic, or round-headed. For the most part, the Old Americans fall into the intermediate class, the average index of males being 78.3 and that of females 79.5. Barring a few French Huguenots, the Old Americans considered here are mostly of British ancestry, and their head form corresponds rather closely to that of the English of the present day. In England, as is well known, the round-headed type of Central and Eastern Europe, the Alpine or Celto-Slav type, has few representatives. The population is composed principally of long-headed peoples, deriving from the two great European stocks, the Nordic and the Mediterranean. To the latter the frequency of dark hair and brown eyes is probably due, both in England and America. While the average of the Old Americans corresponds closely to the average of the English, there is a great deal of variation in both countries. Unfortunately, it is impossible to compare the present Americans with their ancestors, because measurements of the latter are lacking. But to assume that the early colonists did not differ greatly from the modern English is probably justifiable. A comparison of modern Americans (of the old white stock) with modern English should give basis for an opinion as to whether the English stock underwent any marked modifications, on coming to a new environment. It has already been noted that the average cephalic index is practically the same; the only possibility of a change then lies in the amount of variability. Is the American stock more or less variable? Can a "melting pot" influence be seen, tending to produce homogeneity, or has change of environment rather produced greater variability, as is sometimes said to be the case? The amount of variability is most conveniently measured by a coefficient known as the standard deviation ([Greek: s]), which is small when the range of variation is small, but large when diversity of material is great. The following comparisons of the point at issue may be made.[202] Avg. [Greek: s]