ORGANIC GEOLOGICAL DYNAMICS. 641 



sition, had since become extinct, suggested as $, 

 possible cause for this occurrence, that the vital 

 energies of a species, like that of an individual, 

 might gradually decay in the progress of time and 

 of generations, till at last the prolific power might 

 fail, and the species wither away. Such a property 

 would be conceivable as a physiological fact; for we 

 see something of the kind in fruit-trees propagated 

 by cuttings: after some time, the stock appears to 

 wear out, and loses its peculiar qualities. But we 

 have no sufficient evidence that this is the case in 

 generations of creatures continued by the repro- 

 ductive powers. Mr. Lyell conceives, that, without 

 admitting any inherent constitutional tendency to 

 deteriorate, the misfortunes to which plants and 

 animals are exposed by the change of the physical 

 circumstances of the earth, by the alteration of 

 land and water, and by the changes of climate, 

 must very frequently occasion the loss of several 

 species. We have historical evidence of the extinc- 

 tion of one conspicuous species, the Dodo, a bird of 

 large size and singular form, which inhabited the Isle 

 of France when that island was first discovered, and 

 which now no longer exists. Several other species 

 of animals and plants seem to be in the course of 

 vanishing from the face of the earth, even under 

 our own observation. And taking into account the 

 greater changes of the surface of the globe which 

 geology compels us to assume, we may imagine 

 many or all the existing species of living things 



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