350 MANUAL OF THE MOLLUSCA. 



limited to small areas, Avhile others, more widely diffused, unite the local 

 populatious into fewer and larger groups. Those species which characterise 

 particular regions are termed "endemic ;" they mostly require peculiar cir- 

 cumstances, or possess small means of migrating. The others, som.etimes 

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

 of plants propagated by sjwres, and more easily meet with suitable conditions. 

 The space over which a species is distributed is called a " centre," or more 

 propei'ly 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 no-where 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. 56) ; these can only be known approximately in any 

 case. The docti'ine 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 Avell."* 



Generic areas. — Natural groups of species, whether called genera, families 

 01" orders, are distributed much in the same manner as species ;t not for the 



* Mrs Somerville's Phj-sical Geography, II. 95. 



+ " What we call class, order, family, genus, are all only so many names ior genera 

 of various degrees of extent. Technically, a genus is a group to which a name (as 

 Ribes) is applied: but essentially, Exogens, Ranunculace.v] Ranunculus, are genera of 

 different degrees. 



One of the chief arguments in favour of the naturalness of genera (or groups), is 

 that derived from the fact that many genera can he shown to he centralized in definite 

 geographical areas {Erica, for example); i.e. we find the species gathered all, or 

 mostly, within an area, which has some one point where the maximum number of 

 species is developed. 



But, in geographical space, we not unfrequently find that the same genus may 

 have two or more areas, within each of which this phenoinenon of a point of max- 

 unum number of species is seen, with fewer and fewer species radiating, as it were, 

 from it. 



In time, however (or, in other words, in geological distribution), so far as we know, 

 each generic type lias had an unique and continuous range. When once a generic 

 tj'pe has ceased, it never re-appears. 



A genus is an abstraction, a divine idea. The very fact of the centralization of 

 groups of allied species, i.e. of genera, in space and time, is sufficient proof of this. 

 Doubtless we make many so-called genera that are artificial ; but a true genns is 

 natural ; and, as such, is not dependent on man's will." E. Forbes. (Sec An. Nat. 

 Hist. July, 1S52, and Jan. 1S55, p. 45.) 



