GEOGRAPHICAL DISTEIBUTION. 51 



(6) Neotropical or Soutli American region : "West India Islands, 

 South Mexico, Central and South America, Galapagos, and 

 Falkland Isles. These divisions apply to the land, but it is 

 probable that they will help to throw light on the boundaries of 

 the natural marine provinces. 



As will bo seen, each of the above regions includes several of 

 the provinces adopted in this work. 



The Land Provinces hitherto proposed have been chiefly 

 founded on botanical grounds, but the evidence afforded by 

 insects and the higher classes of animals conhrms the existence 

 of these divisions. 



The Marine Provinces have also been investigated by botanists ; 

 and the striking peculiarities of the fisheries have been taken 

 into account as well as the distribution of shell-fish and corals. 



In order to constitute a distinct province it is considered 

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

 rule which applies equally to plants and animals. Some genera 

 and sub-genera are limited to each province, but the proportion 

 is different in each class of animals and in plants.* 



Specific areas. Species vary extremelj' in their range, some 

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

 unite the local populations into fewer and larger groui)s. 

 Those species which characterise i)articular regions are termed 

 "endemic;" they 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 projjerly, a sj)ecific 

 area. The areas of one-half the species are smaller (usually 

 much smaller) than a single province. 



In each sj)ecific area there is frequently one S23ot' where indi- 

 viduals 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 formerly ; 

 and many probably seem rare only because their head-quarters 

 are at present unknown. — {Forbes.) 



Specific 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 (p. 4.6) ; 



* The genera of plants amount to 20,000, and consist on an average of only four 

 .species apiece I The genera of shells commonly admitted are only 400 in number, and 

 average forty species each. It follows that the areas of the moUuscan genera {cceterii 

 paribus) ought to be ten times as preat as those of plants. 



