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147 



tui'kej among birds ; and for these two classes of animals 

 it is probably correctly defined ; but for the terrestrial 

 mollusks, some other division is necessary ; for the most 

 •widely difiused among them does not occupy so large a 

 space, and several groups of species are much more lim- 

 ited, as we have endeavored to show. It will be objected 

 to this theory of distinct zoological centres, that it can- 

 not be natural, because it concerns only one department 

 of animals ; and that, as nature acts through general laws, 

 these separate plans of diffusion are inconsistent with 

 this principle. The objection, in our view, has but little 

 weight ; the laws of nature are known only by their 

 results, and as we see that different classes of animals 

 have different capacities and different powers of action, 

 and of resisting action, we must believe that they were 

 formed to be subject to different influences ; and that 

 their diffusion may, without any inconsistency of purpose, 

 have been regulated by different' plans. And the gen- 

 erally received opinion, of the successive creation of the 

 different classes of animals, after intervals of greater or 

 less durations, adds strong confirmation to our theory ; 

 for, admitting this to be true, the centres from which 

 sprung the various mollusks were established, and the 

 causes influencing their extension and distribution were 

 in operation, for an indefinite period in advance of even 

 the existence of the animals of the higher classes. 



There are difiiculties which it seems to us can hardly 

 be overcome on any other supposition. If all the species 

 had been created at one time and at one place, they 



