52 MANUAL OF TnE MOLLUSCA. 



these can only be known approximately iu any case. Tlie 

 doctrine that each species originated from a single individual, 

 or pair, created once onlj'-, and at one 2)lace, derives strong con- 

 firmation 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."* 



Generic areas. Natural groups of sj)ecies, whether called 

 genera, families, or orders, are distributed much in the same 

 manner as species ; f not for the same reason, since their con- 

 stituents are not related bj' descent, but apparently from the 

 intention of the Creator. 



Sub-generic areas are usually smaller than generic ; and the 

 areas of orders and families are, as a matter of coui'se, larger 

 than those of the included genera. But it is necessary to 

 remember that groups of the same denomination are not always 

 of equal value ; and since species vary in range it often happens 

 that sj)ecific areas of one class or family are larger than generic 

 areas of another. The smallest areas are usually those of the 

 forms termed alerrant ; the typical groups and species are 

 most widely distributed. — {Waterliouse.) 



"When a generic area includes a considerable number of 

 species, there may be found within it a point of maximum 

 [metropolis), around which the number of species becomes less 

 and less. A genus may have more centres than one. It may 

 have had unbroken extension at one period, and yet in the 

 course of time and change, ma 5^ have its centre so broken up 



* Mrs. Somerville's Phj'sical Geography, ii. 95. 



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

 of vai'ious degrees of extent. Technically a genus is a group to which a name (as 

 liibes) is applied : but essentially, Exogens, UanunculacecE , Tianunculus, are genera of 

 different degrees. 



" One of the chief arguments in favour of the naturalness of genera (or f/roups), is 

 lliat derived from the fact that manj' genera can be shown to be centralised in definite 

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

 mostlj^ within an area, which has some one point where tlie maximum number of 

 species is developed. 



" But, in geographical sjmcc, we not unf requently find that the same genus may have 

 two or more areas, within each of which this phenomenon of a point of maximum 

 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 distriljution), so far as we know, 

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

 t}-pe has ceased it never re-appears. 



" A genus is an abstraction, a divine idea. Tlie very fact of the centralisation of 

 groups of allied species, i.e. of genera, in space and time, is sufficient proof of lhi3. 

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

 natural; and, as such, is not dependent on man's -nUl." — E. Forbes. (See .'i.n. Nat, 

 Hist. July, 1S52, and Jan., 1855, p. .45.) 



