Fis MANUAL OF THE MOLLUSCA, 
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 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 species, whether called 
genera, families, or orders, are distributed much in the same 
manner as species; } not for the same reason, since their con- 
stituents are not related by descent, but apparently from the 
intention of the Creator. 
Sub-generic areas are usually smaller than generic; and the 
areas of orders and familes are, as a matter of course, 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 specific areas of one class or family are larger than generic 
areas of another. The smallest areas are usually those of the 
forms termed aberrant; the typical groups and species are 
most widely distributed.—( Waterhouse.) 
‘When a generic area includes a considerable number of 
species, there may be found within it a pomt 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, may have its centre so broken up 
* Mrs. Somerville’s Physical Geography, ii. 95. 
+ “ What we call class, order, family, genus, are all only so many names for genera 
of various degrees of extent. Technically a genus is a group to which a name (as 
Ribes) is applied: but essentially, Hxogens, Ranunculaceae, 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 be shown to be centralised in definite 
geographical areas (Hrica, for example); i.e. we find the species gathered all, or 
mostly, within an area, which has some one point where the mazximwm 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 phenomenon of a point of maximum 
number of species is seen, with fewer and fewer species radiating, as it were, from it. 
“ In time, llowever (or, in other words, in geological distribution), so far as we know, 
each generic type has had an unique and continuous range. When once a generic 
type has ceased it never re-appears. 
“A genus is an abstraction, a divine idea. The very fact of the centralisation of 
groups of allied species, z.c. of genera, in space and time, is sufficient proof of this, 
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 will,”—, Forbes. (See An. Nat, 
Hist, July, 1852, and Jan., 1855, p. 45.) 
