208 NATURAL HISTORY. 



of the species, but only its disappearance temporarily from 

 its original area. 



Lastly, Forbes considered that ( genera,' or natural 

 groups of species, are distributed very much as species 

 are, each genus having its 'generic area.' When a 

 genus includes a large number of species, there may 

 be found within the generic area '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 time, and 

 yet, in the course of time and change, may have its centre 

 so broken up that there shall appear to be outlying 

 points. When, however, the history of a natural genus 

 shall have been traced equally through its extension in 

 time and space, it is not impossible that the area, 

 considered in the abstract, will be found to be necessarily 

 unique.' 



Forbes, in addition, clearly formulated what he termed 

 the ' law of representation ' among species. He showed, 

 namely, that in all regions, however widely removed from 

 one another, species or groups of species of animals are 

 found which are very like each other, provided only that 

 the conditions of these regions are similar as regards 

 climate. In other words, wherever similar life-conditions 

 prevail, similar species of animals will be found. In all 

 such cases, however, though the species of such regions 

 are similar, they are not identical. Such species he termed 

 ' representative species.' 



As, in Forbes's view, species are permanent and 

 immutable, the only explanation which he could give 

 of the existence of these representative forms was that 



