E. V. Wulff —112— Historical Plant Geography 



down and the newly introduced vegetation receives a habitat with rich, 

 unexhausted soil and without competitors. But even in such cases the 

 invaders ordinarily maintain their ascendancy only under the protection 

 of man, who does not allow the indigenous vegetation to regain its 

 usurped rights. But once man deserts any such place of habitation, 

 the indigenous vegetation with amazing rapidity returns to its own, 

 this being particularly striking in forest regions, where abandoned culti- 

 vated land and settlements are again so densely covered with woods as 

 to be unrecognizable. 



Views formerly held that introduced species (aliens) may completely 

 crowd out the indigenous vegetation have, after detailed studies of the 

 resultant interrelations, been discarded as erroneous. Thus, Allan 

 (1936) points out that in the flora of New Zealand out of about 600 

 aliens only 48 may be regarded as serious competitors of the indigenes. 

 These 48 include 28 from the Old World, 7 from Australia, 9 from the 

 Americas, and 4 from South Africa. These species have a localized 

 distribution limited to formations modified by man and under his 

 constant protection, e.g., pastures. If such vegetation had been com- 

 pelled to develop independently, without man's protection, the in- 

 digenes would, undoubtedly, have crowded out the aliens. Allan 

 concludes: "We are left with the result that only some half dozen 

 aliens can truly be said to have suppressed any indigene^and that 

 very locally" (p. 191). 



Similar and extremely interesting data have been reported by 

 Perkier de la Bathie (1932) for Madagascar. The total number of 

 species that have been introduced into Madagascar accidentally or 

 intentionally by man is 524. To this number we must add 380 species 

 alien to the local flora, which reached the island as a result of natural 

 factors of dispersal or the manner of whose entry is unknown but which 

 are distinguished by the same characters as the flora introduced by 

 man: absence in indigenous plant communities, occurrence in open 

 associations or in those modified by man, possession of characters 

 alien to the local flora, high generic coefficient (ratio of number of 

 genera to number of species), seeds whose wide dissemination is 

 readily possible. Consequently, the total number of species alien to 

 the flora of Madagascar is approximately 900, a very large number, but 

 this is not surprising when we recall that about 70 per cent of the 

 natural vegetation of Madagascar has been destroyed by man. Of all 

 these 900 species, however, only one, Adenoslemma viscosum, has gained 

 a foothold in natural plant communities undisturbed by man. This is 

 an exceptionally remarkable and important fact. 



As long ago as 1899 Flahault emphasized the fact that the boun- 

 daries of the area of a species depend on environmental conditions to 

 a far greater degree than had been supposed and that the slightest 

 deviation in these conditions may threaten the existence of a species, 

 may even cause it to suffer annihilation in the struggle for existence. 

 This explains why in many cases adventive vegetation, apparently 

 fully naturalized, ceases to bear fruit and to spread independently as 

 soon as man leaves it to itself and deprives it of his direct or indirect 

 protection. 



