692 NOTES TO BOOK XVIII. 



own hypothesis of the introduction of new species upon 

 the earth, not having any physiological basis, hardly 

 belongs to this chapter. 



Mr. Lyell has explained his theory (B. in. c. viii. p. 1 66) 

 by supposing man to people a great desert, introducing 

 into it living plants and animals ; and he has traced, in a 

 very interesting manner, the results of such a hypothesis on 

 the distribution of vegetable and animal species. But he 

 supposes the agents who do this, before they import species 

 into particular localities, to study attentively the climate 

 and other physical conditions of each spot, and to use 

 various precautions. It is on account of the notion of 

 design thus introduced that I have, in the text, described 

 this opinion as rather a tenet of Natural Theology than of 

 Physical Philosophy. 



Mr. Edward Forbes has published some highly inte- 

 resting speculations on the distribution of existing species 

 of animals and plants. It appears that the manner in 

 which animal and vegetable forms are now diffused re- 

 quires us to assume centers from which the diffusion took 

 place by no means limited by the present divisions of con- 

 tinents and islands. The changes of land and water 

 which have thus occurred since the existing species were 

 placed on the earth must have been very extensive, and 

 perhaps reach into the glacial period of which I have 

 spoken in note* (EA). See, in Memoirs of the Geological 

 Survey of Great Britain, vol. i. p. 336, Professor Forbes's 

 Memoir " On the Connexion between the Distribution of 

 the existing Fauna and Flora of the British Isles, and the 

 Geological Changes which have affected their area, espe- 

 cially during the epoch of the Northern Drift. 1 ' 1 



According to Mr. Forbes's views, for which he 



