GEOGRAPHICAL DISTRIBUTION. 51 
(6) Neotropical or South 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 be 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 confirms the oxistence 
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 hmited 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 characterise particular 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 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 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.—(Lorbes.) 
Specific centres are the points at which the particular species 
are supposed to have been created, according to those who 
belieye that each has originated from a common stock (p. 46) ; 
* The genera of plants amount to 20,000, and consist on an average of only four 
species apiece! The genera of shells commonly admitted are only 400 in number, and 
average forty species each. It follows that the areas of the molluscan genera (ceteris 
paribus) ought to be ten times as great as those of plants, 
