GEOGRAPHICAL DISTRIBUTION. 851 



same reason, since their coustitneuts 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 families are as a matter of course larger than those of the included 

 genera. But it is necessary to remember that groups of the same denomi- 

 nation 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 gcner ic 

 ai-eas of another. The smallest areas are usually those of the forms termed 

 aberrant (p. 61) ; the tyjucal groups and species are most widely distrib uted. 

 ( Water Jiouse.) 



" 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 tlie 

 course of time and change, may have its centre so broken up that there shall 

 appear to be out-lying points. When, however, the history of a natural 

 genus shall have been traced equally through its extens'ion in time and space, 

 it is not impossible that the area, considered in the abstract, will be found to 

 be necessarily unique." {Forbes) 



To illustrate the doctrine of the uniti/ of generic areas Prof. Forbes has 

 given several examples, showing that some of the most exceptional cases 

 admit of explanation and confirm the rule. One of these relates to the genus 

 Ultra of which there are 400 species; it has its metropolis in the Philippine 

 Islands and extends by the Red Sea to the ^Mediterranean and West Africa, 

 the species becoming few, small, and obscure. Far away from the rest a 

 single species is found on the coast of Greenland ! But this very shell occurs 

 fossil in Ireland along with another r^iitra now living in the Mediterranean. 

 Another case is presented by the genus Fanopaa, of which the six living 

 species are widely separated, — a, in the ^lediterranean ; b, in Patagonia ; 

 c,dX the Cape; d, Tasmania; e. New Zealand;/, Japan. Of this genns 

 above 100 fossil species are known, distributed over many places within the 

 wide area, on whose margin the relics of this ancient form of life seem to 

 linger, like the last ripple of a circling wave.* 



According to this view the specific centres are scattered thickly over the 

 whole surface of the globe ; those of the genera more thinly distributed ; and 

 the points of origin of the large groups become fewer in succession, m;ti! w- 

 have to estimate the probable position or scene of creation of the priuiaiy 

 divisions themselves ; and are led to speculate whether there may not have 

 been some common focus— the centre of centres— from which the first and 

 greatest types of life have emanated. 



Boundaries of Natural History Frovinces. The land provinces are sepa- 



* The most striking and conclusive instances may be met with in the distribution 

 of the higest classes of vertebrate animals. 



