154 GEOGRAPHICAL DISTRIBUTION OF THE MOLLUSCA. 



blage or congeries of species as evidence of the existence of 

 favoring- conditions, and the inference tliat therefore those 

 species are to be found nowhere else. It was thought that tlie 

 conditions which prevented the JSTeptuneas of the boreal seas 

 from extending southwards of Cape Cod, also would prevent 

 the extension southwards of the equally boreal genus Buccinuni, 

 yet a typical species of the latter genus has far overstepped the 

 boundary ; and many species go wandering about the world like 

 knights-errant, challenging the theorists — who invariably decline 

 the issue on the ground that finding them where they have no 

 business to be is sufficient proof that they are not themselves. 

 In order to constitute a distinct province it is considered 

 necessary that at least one-half the species should be peculiaf^, a 

 rule which applies equally to plants and animals. Some genera 

 and subgenera are limited to each province, but the proportion 

 is different in each class of animals and in plants. 



Specific Areas. Species vary extremely in their range, some 

 being limited to small areas, while others, more widely diffused, 

 unite the local populations into fewer and larger groups. 

 Those species which characterize particular regions are termed 

 "endemic;" thej'^ mostly require peculiar circumstances, or 

 possess small means of migrating. The others, sometimes called 

 " sporadic," possess great facilities for diffusion, like the lower 

 orders of plants propagated by spores., and more easily meet with 

 suitable conditions. The space over which a species is distributed 

 is called a " centre," or, more properly, a specific area. The 

 areas of one-half the species are smaller (usually much smaller) 

 than a single province. 



In each specific area there is frequently one spot where 

 individuals are more abundant than elsewhere ; this has been 

 called the " metropolis " of the species. Some species which 

 appear to -be nowhere common can be shown to have abounded 

 formerl}^ ; and many probably seem rare only because their 

 headquarters are at present unknown. — (Forbes.) 



Specif c Centres are the points at which the particular species 

 are supposed to have been created, according to those who 

 believe that each has originated from a common stock ; these 

 can only be known approximately in any case. The doctrine 

 that each species originated from a single individual, or pair, 

 created once only, and at one place,. derives strong confirmation 

 from the fact that so " many animals and plants are indigenous 

 only in determinate spots, while a thousand others might have 

 supported them as well." 



Boundaries of Natural History Provinces. The land provinces 

 are separated by lofty mountains, deserts, seas, and climates ; 

 whilst the seas are divided by continents and influenced by the 

 physical character of coast-lines, by climates and currents. 



