WILD NATURE. 341 



change at every succession ; that, calling the average 

 duration of the domestic animal ten years, and that 

 of the bread-plant one year, is three hundred suc- 

 cessions 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 impossi- 

 bility 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 thou- 

 sand 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 in- 

 formation upon, not a few only, but on many points, 

 much of the change must have been theoretical ; and, 

 like all theoretical procedure, sometimes an improve- 

 ment, and sometimes the reverse. But there is an- 

 other difficulty. When great changes are made on 

 the surface of a country, 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 produc- 

 tions 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 spe- 

 cific characters admit, and the introduction of not 

 varieties only, but of species altogether new. 



That not only may, but must have been the case. 

 The productions of soils and climates are as varied 

 as these are ; and when a change takes place in 

 either of these, if the living productions cannot alter 

 their habits so as to accommodate themselves to the 

 change, there is no alternative, but they must perish. 

 Also, though we know nothing about the primary 

 Ff2 



