80 INVASION 



portance, as it enables us to retrace the steps by which the 

 formation has reached its present structure, and to recon- 

 struct formations long since disappeared. To render it less 

 difficult, it is necessary to scrutinize the derived elements 

 closely, first, because it is easiest to recognize the indigenous 

 species by eliminating the derived, and second, because this 

 analysis will show that not all derived species have entered 

 the formation at the same time and from the same sources. 

 Watson (1847:63) distinguished plants as colonist, alien, den- 

 izen and native, the last corresponding to indigenous. 

 Colonists are weeds of fields and wastes, rarely found out- 

 side of places modified by man ; aliens are plants which, 

 though now more or less established, have been certainly 

 or presumably introduced from foreign counries, while den- 

 izens are species which maintain themselves as though in- 

 digenous, but are suspected of being introduced. A. De- 

 Candolle (J855:643) made the following division: (1) advent- 

 ive, of foreign origin, but only temporarily established; (2) 

 naturalized, established but certainly of foreign origin ; (3) 

 probably foreign; (4) possibly foreign; (5) indigenous, of 

 undoubted native origin. Pound and Clements (1900:52) dis- 

 tinguished regional elements as proper (indigenous), de- 

 rived, and adventicious. Neither of these three groupings 

 gives perfectly definite and consistent divisions, and for this 

 reason the following arrangement is proposed. With res- 

 pect to their original formation or region, species are 

 either indigenous, native, or derived, foreign. Derived 

 species may be termed v i c i n e {species vicinae), when they 

 are fully established invaders from adjacent formations or 

 regions, and adventicious (species adventiciae), when they 

 have come from distant formations and have succeeded in 

 establishing themselves. Finally, those derived species 

 which are unable to establish themselves permanently are 

 adventive {species adventivae). 



THE MANNER OF INVASION. 



Since the ecesis of invaders depends in large measure up- 

 on the occupation of the plants in possession, the method 

 and degree of invasion will be determined by the presence 



