著者

徳田 御稔

出版者

社団法人日本動物学会

雑誌

動物学雑誌 (ISSN:00445118)

巻号頁・発行日

vol.59, no.9, pp.210-213, 1950-09-15

被引用文献数

3

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In reference to the distribution and territories occupied by certain closely-related species, Mayr (1947) has proposed the use of the terms "sympatric" and "allopatric." The former applies to those species which occupy the same geographical areas; the latter to those species which occupy separate geographical areas. In the case of "sympatric" species there exist successive grades of coincidence in the territories occupied ranging from perfect coincidence of territoy to the maintenance of only a small overlapping margin. In any case there must be a cortain biological gap or "break" among the "sympatric" species which prohibits them from interbreeding. Taxonomically good species frequently hold this type of distribution. On the other hand, in the case of "allopatric" species we find those which are only maintained by geographical isolation. At least theoretically there exist two formerly interbreeting colonies or incipient species siolated from each other rather recently so that they will easily interbreed if the barriers are withdrawn. We can find a number of "allopatric" examples in Japan, especially among the related species isolated from each other by Tsugaru-strait, so that while one form may be restricted to Hokkaido another is confined to Hondo. Some of them are good species so far as judged by morphological differences, while others belong to 'not good species' maintained only by the geographical barrier. Our interests are more profound in the case of "allopatric" species without geographical isolation, since in this case there must exist some other intrinsic reasons for their separation within the distributional range. In the book of Mayr we find examples of this type of "allopatric" species mainly in mammals and birds. While in lower forms, for example in European toads and frogs, Mayr stressed discovery that the two related forms frequently produce hybrids at a few marginal regions where ranges of each species overlap. In the higher forms of vertebrates there seems to exist some psychological reason to prohibit the crossing of species in the overlapping zone, even though they can be hybridized easily in captivity. At any rate the "allopatric" species present problems which require more considerable research in the wide field of biological study. I have shown in the present report two distinct cases of "allopatric" species of small mammals revealed by the comparative study of their distribution according to elevation on Mt. Yatsugatake and Mt. Ontake. Both mountains are situated in central Hondo within the same bio-geographical area. Two species of Clethrionomys (andersoni=Ca and smithii=Cs) and two species of Urotrichus (talpoides=Ut and pilirostris=Up) have been the subjects of my study. In figs. 1 and 2, I have shown the exact location of the stations where I trapped these species. It seems remarkable that Clethrionomys smithii which is restricted to the lower slopes on Mt. Yatsugatake, expands its range higher than 2000m. on the eastern slope of Mt. Ontake where other rival species are absent because of the lack of favourable conditions.