In zoology, related species when grouped together constitute subgenera and genera and the term species implies the existence of a certain definite amount of divergence from the most closely related type but race does not require a similar amount of difference. In man, where all groups are more or less fertile when crossed, so many intermediate or mixed types occur that the word species has at the present day too extended a meaning. For the sake of clearness the word race and not the word species or subspecies will be used in the following chapters as far as possible. The old idea that fertility or infertility of races of animals was the measure of species is now abandoned. One of the greatest difficulties in classifying man is his perverse predisposition to mismate. This is a matter of daily observation, especially among the women of the better classes, probably because of their wider range of choice. There must have existed many subspecies and species, if not genera, of men since the Pliocene and new discoveries of their remains may be expected at any time and in any part of the eastern hemisphere. The cephalic index is of less value in the classification of Asiatic populations but the distribution of round and long skulls is similar to that in Europe. The vast central plateau of that continent is inhabited by round skulls. In fact, Thibet and the western Himalayas were probably the centre of radiation of all the round skulls of the world. In India and Persia south of this central area occurs a long skull race related to Mediterranean man in Europe. Both skull types occur much intermixed among the American Indians and the cephalic index is of little value in classifying the Amerinds. No satisfactory explanation of the variability of the skull shape in the western hemisphere has as yet been found, but the total range of variation of physical characters among them, from northern Canada to southern Patagonia, is less than the range of such variation from Normandy to Provence in France. In Africa the cephalic index is also of small classification value because all of the populations are characterized by a long skull. The distinction between a long skull and a round skull in mankind probably goes back at least to early Paleolithic times, if not to a period still more remote. It is of such great antiquity that when new species or races appear in Europe at the close of the Paleolithic, between 10,000 and 7,000 years B. C., the skull characters among them are as clearly defined as they are to-day. The fact that two distinct species of mankind have long skulls, as have the north European and the African Negro, is no necessary indication of relationship and in that instance is merely a case of parallel specialization, but the fact, however, that the Swede has a long skull and the Savoyard a round skull does prove them to be racially distinct.