222 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



there was too humid for its normal biological development, the 

 pistillate stage of each flower being over by the time it opened. As 

 has been pointed out by Wulff in the work cited at the end of this 

 chapter, if to such precise needs ' we add the unceasing struggle 

 for existence and the competition with the indigenous vegetation, 

 we should not be surprised at the relatively small number of those 

 species introduced by man for cultivation or accompanying him in 

 his migrations that became fully naturalized components of the local 

 flora '. As an outstanding example it may be mentioned that, of 

 the nearly one thousand alien species in the flora of Madagascar, 

 it is claimed that only one, which is particularly easily dispersed, 

 has gained a foothold in plant communities undisturbed by Man. 

 Only such territory may be considered as ecologically fully occupied. 



For cultivation, land has in general to be cleared of native vegeta- 

 tion and otherwise specially prepared. Such ousting of the native 

 flora gives the adventives a chance to spread; so does less complete 

 destruction of the vegetation, for example by domesticated animals. 

 But once the cultivation or other disturbance is discontinued, there 

 ensues a struggle between the alien and native plants which usually 

 ends in victory for the latter and return to approximately the original 

 condition. This is particularly noticeable and rapidly effected in 

 the more favourable forested regions, whereas in some others, such 

 as the drier grasslands, the breaking of the sod or other disturbance 

 may so upset the ecological balance as to make its return extremely 

 slow or even problematical. Another interesting example of this 

 appears to be aflforded in some of the most favourable situations in 

 southwestern Greenland. Here the clumps of Willows in many 

 valleys are nowadays separated by grassy tracts {cf. Fig. ii6) much 

 as they presumably were at the time of the extinction of the Viking 

 colonies and their pasturing Sheep several centuries ago : at least, 

 the present writer has been unable, during hundreds of miles of 

 wandering in those now uninhabited regions, to think of any other 

 explanation of a remarkable phenomenon. Nor have the tree 

 Birches returned at all widely, either in those parts of Greenland 

 or in Iceland, since the ' forests ' were decimated in the early centuries 

 of the present millennium. 



The majority of really widespread weeds, such as those which 

 qualify as semi-cosmopolites, tend to be collective species (such as 

 the Common Dandelion, Taraxacum officinale s.l.) or to consist of 

 numerous races (as in the Couch-grass, Agropyion repens) adapted 

 to diverse habitat conditions. The distinction between these two 



