File:Age and area; a study in geographical distribution and origin of species (1922) (14798541433).jpg

Identifier: ageareastudyinge00will_0 (find matches)
Title: Age and area; a study in geographical distribution and origin of species
Year: 1922 (1920s)
Authors: Willis, J. C. (John Christopher), 1868-1958
Subjects: Biogeography Evolution
Publisher: Cambridge (Eng.) The University press
Contributing Library: Wellesley College Library
Digitizing Sponsor: Boston Library Consortium Member Libraries

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tralAmerica and the West Indies. The rest, with 20, 12, 9 and 3species, cover smaller ranges within these. Or if one take the large and widespread Menispermaceae (fig.on p. 173), one finds (37) Cocculus and Cissampelos with adistribution practically covering that of the family, Stephaniaand Tinospora covering most of the Old World, and Hyperbaenamost of the New World, range. Within these are many generaof smaller and smaller range till one comes down to the 12in West Africa, 5 in Brazil, 5 in Madagascar, etc. There are2 genera of maximum range, 4 of rather less (including Meni- 1 This curious point, that the most widely spread genus of all has fewerspecies than some of the others, is by no means unique, but occurs in anumber of families, e.g. also in the Menispermaceae, Cistaceae, and Hydro-phyllaceae. It requires careful investigation with the aid of paleobotany,for it seems to me not impossible that the deficiency in species may beconnected with the occurrence of the glacial period.
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174 ENDEMISM AND DISTRIBUTION: GENERA (pt. ii spermum in Atlantic North America and north-east Asia), about19 in the next class, and 24 or more in the lowest class. This type of distribution corresponds to that of the species ofDoona, Gymnema, Cissampelos, etc., described in the precedingchapter (p. 157). But the Cyrtandra type (p. 159) can also bematched, e.g. by the family Monimiaceae (37), in which thereare 22 genera with small areas (the largest being New Guinea andCelebes) and 49 species in all (average 2-2 species per genus),5 genera with areas of moderate size (and 22 species, average 4-4),and 5 with areas of large size (and 196 species, average 39-2).These larger areas overlap one another to some extent in somecases, but there is no single genus covering, or nearly covering,the range of the family. All these groups of genera, it will be seen, give indications, evenwhen considered singly, that the areas they occupy go with theirnumber of species, and if taken in groups, the app

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