WILD NATURE. 369 



cession ; that, calling the average duration of the 

 domestic animal ten years, and that of the bread- 

 plant one year, is three hundred successions in 

 the one case, and three thousand in the other ; 

 and what man is to live, nay, what kingdom is to 

 last till the experiment is performed as many times, 

 with any thing which is now in a state of nature ! 

 Even if we were to suppose that impossibility got 

 the better of, there arises another every way as 

 perplexing. How are we to know what was the 

 first artificial mode of treating any one of those 

 cultivated productions, or what were the effects of 

 it, even at the end of one thousand, or of two 

 thousand years ? Fashions of cultivation change 

 as much as fashions of any thing else ; and as the 

 subject is one in which it is impossible to get 

 accurate information upon, not a few only, but on 

 many points, much of the change must have been 

 theoretical; and, like all theoretical procedure, 

 sometimes an improvement, and sometimes the 

 reverse. But there is another difficulty. When 

 great changes are made on the surface of a coun- 

 try, as when forests are changed into open land, 

 and marshes into corn fields, or any other change 

 that is considerable, the changes of the climate 

 must correspond ; and as the wild productions are 

 very much affected by that, they must also undergo 

 changes ; and these changes may in time amount 

 to the entire extinction of some of the old tribes, 

 both of plants and of animals, the modification of 

 others to the full extent that the hereditary specific 

 characters admit, and the introduction of not varie- 

 ties only, but of species altogether new. 



That not only may, but must have been the 

 case. The productions or soils and climates are 



