INTRODUCTION ix 



are considered to be aliens for geographical reasons. 

 It is a general experience, and one that is to be expected^ 

 that two areas, inhabited by a given native species, 

 are seldom known to be separated by a large tract 

 of similar, and apparently suitable, ground devoid 

 of that species. Where such a gap exists it is often 

 found that the indigenous character of the species 

 in one or other of the areas is for some reason 

 under suspicion. It is, of course, not impossible that 

 such a gap might occur in the truly native range of 

 a species, in consequence of its approaching extinction 

 over the whole region, or from some other cause» 

 Great caution has therefore been exercised in applying 

 the argument of discontinuity to discredit the nativity 

 of a species. The study of geographical distribution 

 becomes chiefly useful when combined with an exami- 

 nation of the varying habitats of a species over large 

 areas ; it can then hardly fail to throw light on the 

 status of the plant in the different parts of its total 

 range. The total range of any plant comprises all 

 the countries in which it occurs, and, in cases where 

 artificial dissemination has enlarged the original area, 

 the total range may sometimes be satisfactorily divided 

 into concentric zones corresponding to the increasing 

 dependence of the plant on man as it recedes from 

 its native centre. Thus a number of species which 

 are native in the Mediterranean area are known further 

 north on the Continent as established weeds, but reach 

 England only as casuals. The gradation in these cases 

 may be attributed to climate. Again, some of the 

 natives of the Orient have overspread Europe as weeds 

 of agriculture, having very probably accompanied the 

 different incursions of the human race from east to- 

 west in prehistoric times. For the purposes of this 

 list, then, a species will not be considered a native of 

 Britain which is not known in at least one natural 

 habitat, nor even then if it can be shown, by geographical 

 or other arguments, that it was introduced thither by 

 artificial means or from an artificial source. A species 



