576 HISTORY OF GEOLOGY. 



nerian Society for 1837-8, must be esteemed the foundation of a criti 

 cal investigation of this subject in Britain.] 



2. Extinction of Species. With regard to the extinction of species, 

 Mr. Lyell has propounded a doctrine which is deserving of great atten- 

 tion here. Brocchi, when he had satisfied himself, by examination 

 of the Sub-Apennines, that about half the species which had lived at 

 the period of their deposition, had since become extinct, suggested as 

 a 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 pro- 

 pagated 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 seve- 

 ral species. We have historical evidence of the extinction of one con- 

 spicuous species, the Dodo, a bird of large size and singular form, 

 which inhabited the Isle of France when that island was first disco- 

 vered, and which now no longer exists. Several other species of ani- 

 mals 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 to be extirpated. If, for instance, that reduction of 

 the climate of the earth which appears, from geological evidence, to 

 have taken place already, be supposed to go on much further, the 

 advancing snow and cold of the polar regions may destroy the greater 

 part of our plants and animals, and drive the remainder, or those of 

 them which possess the requisite faculties of migration and accommo- 

 dation, to seek an asylum near the equator. And if we suppose the 

 temperature of the earth to be still further reduced, this zone of now- 

 existing life, having no further place of refuge, will perish, and the 

 whole earth will be tenanted, if at all, by a new creation. Other 

 causes might produce the same effect as a change of climate ; and, 

 without supposing such causes to affect the whole globe, it is easy to 



