XIX 



such plants, had they been originally created in the coun- 

 tries in which, when imported, they thus prevail, could have 

 been extirpated subsequently by the unfavourable influence 

 of external conditions. 



We are driven then, by a process of logical exhaustion, 

 to conclude that each species was originally introduced into 

 a particular locality, from whence it diffused itself over a 

 greater or lesser area, according to the amount of obstacles 

 which checked its propagation, and its own inherent power 

 of surmounting them. 



To this theory sufficient allusion has been made in the 

 body of the Work, so that all I need do is to anticipate cer- 

 tain difficulties which may appear to stand in the way of 

 its application to particular cases, and to point out the in- 

 ferences with respect to the former condition of the globe 

 which these facts serve to elicit. 



In the first place then, I believe it must be admitted, 

 that, after attaching all due importance to the curious con- 

 trivances so ably put before us by De Candolle,* through 

 which Nature has in many instances provided for the dis- 

 semination of species, numerous cases will present them- 

 selves of a very embarrassing nature, both as regards the 



* Essay on the Geography of Plants, Diet. d'Histoire Naturellc. 



