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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ime is allowed, and no other agency, such as man, interferes, byshrub and forest. Incidentally, a method of dispersal which has not been men-tioned above must receive a word of notice. This is the explosivemechanism, as it is sometimes called, where, owing to tensionsset up in the fruit by turgidity, as in Impatiens, or by drying,as in Claytonia, Montia, Hevea, Hura, etc., the seeds when ripeare jerked away from the plant. The distance is commonly quitesmall, but when, as in Hura or Heir a, the fruits are at the topof a tall tree may be slightly increased. In many respects, the last regular mechanism which has tobe mentioned, that of vegetative reproduction by portions ofthe plant itself, like runners, suckers, bulbils, etc., is the mostefficient of all, as witness the profusion of daisies in most lawns,or the difficulty of eradicating Jerusalem artichokes once estab-lished; while anyone who has had the misfortune to have hisgarden infested with goatweed, enchanters nightshade, celan-
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18 THE DISPERSAL OF PLANTS (pt. i distance from the mainland, but as regards frequency of arrivalof species from elsewhere. There are few local island speciesamong the beach plants, which are continually arriving with theocean currents, more among the mountain-top plants, whereprobably birds most commonly alight on arrival, and mostamong those of intermediate elevation. He regards as the oldest, on the whole, those groups withactual genera confined to the island or group of islands, thenthose with genera all of whose species are endemic, followed bythose having genera with some species endemic and some widelydistributed, and as the youngest, on the whole, those havingonly genera with no species endemic. He regards the develop-ment of endemic species as due to what he calls the principle ofdifferentiation. They are most often allied to some commonwidely ranging and polymorphous species which he regards asthe parent. To this very important conclusion he returns inother papers (45-6), an

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