There are non-somatometric characteristics whose distribution pattern show clines, such as the gene frequencies of the ABO blood group (Nei and Imaizumi, 1969), frequencies of fingerprint pattern types (Mitsuhashi, 1972; Iwamoto, 1980) and facial flatness of the cranial series for recent times as well for the Jomon Period (Yamaguchi, 1980). This suggests the possibility that the pattern of geographic variations in somatometric data may turn out to be a dine.

Analysing the data from the Nishi series, I found that many measurement items in fact showed clines, and that the latter could be explained by a cline in body size. I also found that body size, temperature and intake of nutrients were highly correlated, and that the cline in body size can be explained in terms of body heat regulation, namely Bergmann's rule, via food intake.

On the other hand, cephalic index was highly correlated with dietary pattern, and showed a kind of concentric circular distribution pattern: a more brachycephalic group distributed in the central part of Japan (Kinki group), and a less brachycephalic group in the peripheral part (Peripheral group). This pattern has been previously emphasized as a clue to the racial background of modern Japanese. For example, Kohama considered this cephalic index distribution pattern to stem from the fact that the inhabitants of Japan are a racial mixture of dolichocephalic proto-Ainu and brachycephalic Koreans (Kohama, 1960). However, a concentric circular pattern is at present observable only in cephalic index, and several assumptions must be proved before this theory can be accepted. These assumptions include the following: 1) the cephalic index is strongly controled by genetic factors and unchangeable by environmental factors; 2) the distribution pattern was established about 2000 years ago, and has not changed basically since then; and 3) geographic difference between Japanese local populations is as great as inter-racial difference.

Investigations on immigrants to the United States and Hawaii (Boas, 1913; Hirsh, 1927; Shapiro, 1939; and others), on secular change in historic times (Weidenreich, 1945; Suzuki, 1969) and on the genetics of somatometric measurements using data for twins (Suzuki and Ehara, 1956; Osborne and DeGeorge, 1959; Vandenberg, 1962; and others) have revealed that cephalic index is not as stable with respect to environmental factors as had been thought. The pesent study also revealed that Japanese ecame considerably taller and more brachycephalic dring the 30 years from the 1910s to the 1940s. Consequently, as already pointed out by Weidenreich (Weidenreich, 1945), theories of inter-racial relationships should not be advanced solely on the basis of cephalic index data.

The results of the present study may be taken as evidence against stability in the distribution pattern of the cephalic index, since they indicated the possibility that the rate of secular change in physical characteristics was different for the Kinki and Peripheral groups. The results of the present study tell nothing about geographic variation before the collection of the data in the Matsumura series in the 1910s, However, they do suggest that, because the rate of secular change was different in the two groups, geographic variation has probably not remained unchanged throughout the last 2000 years. Therefore, secular change should be taken into account when considering geographic variation in the cephalic index.

Secular change in cranial characteristics and stature from the Jomon people (prehistoric hunter-gatherers) to the present day Japanese has been investigated by Suzuki (1969) and by Hiramoto (1972). They found that the changes in these characteristics were continuous, but that the rate of change was not constant. Suzuki pointed out that the two periods of rapid change have been the Yayoi age (prehistoric early farming period) and recent times. The former corresponds to a transitional period when a new subsistence technology was established, and the latter to the transitional period between the feudal Tokugawa period and the modern industrial era. The materials used in the present study belong to the second period of rapid change.

The causes of secular change in head shape or stature have not yet been clarified, though there are three opinions about the main cause: 1) hybridization with other racial groups (Janz, 1973), 2) heterosis derived from a decrease in the frequency of consanguineous marriages (Schreider, 1967; and others), and 3) environmental factors such as better nutrition and improved medical care (Damon, 1965; Olivier, 1980).

In the case of the change between the Matsumura and Nishi series, which took place over about 30 years, the first alternative is most unlikely. The second alternative is also unlikely, since a period of 30 years, or one generation, seems too short to bring about a nation-wide change in the gene pool, especially in the peripheral areas. The existence of heterosis in Japan is yet to be proved. On the other hand, investigations on Japanese immigrants to Hawaii or the United States have revealed that the effect of environmental factors on somatometric measurements was large (Suski, 1933; Shapiro, 1939; Ito, 1942; Gleulich, 1957; Miller, 1961; Froelich, 1970). Consequently, the third alternative seems to be the most likely cause of secular change, at least in the present case.

If secular change is mainly due to the environmental factors, it is not unreasonable to assume that the rapid change of recent times first took place in the Kinki district, which has been culturally advanced throughout historic times. The present results suggest that this secular change is probably related to change in food intake. The fact that the features which have been considered to be peculiar to the population of Kinki district-tall height and brachycephaly-coincide with those of the advanced type in secular change is compatible with the following hypothesis: geographic difference in cephalic index (most clearly observed in the Matsumura series and least clearly in the Nishi series) was brought about by the rapid secular change which began in the later Edo era, with an early centre in the Kinki district. When the rate of change began to slow down in these areas, change proceeded rapidly in the peripheral areas. The homogenization of physical traits which occurred between the Matsumura and Nishi series may relate to the improvement in transportation facilities and to the increased population movement which has taken place since the Meiji era. In sum, it is likely that the main cause is homogenization in culture and living conditions rather than homogenization of the gene pool.

Even if the geographic difference of the Matsumura series came into existence by the above process, this does not necessarily imply the absence of geographic difference before that time. At present, the quantity of skeletal materials available to researchers is not sufficient to permit verification of hypothesis concerning geographic variations in protohistoric and historic times. The hypothesis which explains the distribution pattern of the cephalic index by geographic differences in the timing of the start and the rate of the rapid secular change in recent times will be proved or disproved when sufficient skeletal materials are collected, for this problem cannot be solved by analysis of somatometric data. The basis of the hypothesis would be weakened should some other characteristic with a high degree of genetic control be found to show a concentric circular distribution pattern like that observed for the cephalic index.

The present results indicated that all Japanese groups were similar to Eleut, Dahur, Manchu, North Koreans, North Chinese, Bunun and Atayal, and were different from Buriat, Orochon, Lao, Thai and Ainu. The conclusion that Japanese are different from Ainu is apparently inconsistent with results obtained in studies on dentition (Hanihara, 1978; Hanihara, et al., 1975) and biochemical characteristics (Omoto, 1978; Omoto and Harada, 1975). However, the contradiction may be more apparent than real, because the present study was based only on Asian Mongoloid groups. Since biological distances represent a relative and not an absolute similarity, it is expected that if Caucasoid or Negroid groups were also included in a somatometric comparison, the distance between Ainu and other Mongoloid groups, including Japanese, would become relatively smaller.

Modern Japanese are similar to the racial groups of northern Manchuria, which have been considered to be highly cold-adapted (Coon, et al., 1950). However, since they are also similar to Formosan aborigines, who arc not characterized by cold adaptation, their ancestors probably included Mongoloid groups not yet characterized by a high degree of cold adaptation. Since Japanese are different from populations of Southeast Asia such as Burmans and Thai, contact with these populations may not be very important. Similarly, a connection between Japanese and Ainu cannot be emphasized.

Based on the above points, I reached the following conclusions concerning the geographic variation and phylogeny of modern Japanese:

1. The concentric circular distribution pattern in cephalic index seems to be explained by geographic differences in the timing of the start and the rate of the rapid secular change in recent times; namely, the change has first started in Kinki district, which has throughout historic times been a culturally advanced area.
2. The explanation of the geographic variation in cephalic index proposed by Kohama is unacceptable for the following reasons: (a) the cephalic index is rather easily influenced by environmental factors; (b) the pattern of geographic variation observed in modern Japanese may not be the same as that in the past because of geographic difference in the rate of secular change; (c) geographic difference between local populations in Japan is not considered to be great enough to support the heterogeneity of Japanese; (d) all the Japanese groups are similar to the same racial groups in Penrose's shape distance, which was found to be the most reliable of the measures of inter-population relationship used in the present study; (e) the relation between modern Japanese and Ainu is not very close.
3. The primary phylogenetic relationship with the modern inhabitants of Japan may be found in those groups of Mongoloids which have not been extremely specialized for adaptation to cold. Relations to Ainu or Mongoloids of Southeast Asia, are not considered to be important, at least as far as the somatometric data for the modern populations in Japan are concerned.