CH. Ill] SPREAD OF FOREIGN SPECIES 27 



of Europe. In actual fact, however, man, by felling the forest 

 ntroduemg cattle, and in other .vays, has completei? altered the 



InTv Te r^H ^•"•''''" ^''' P- ''^ ''^* introduction: 

 only give the characteristic stamp to the vegetation "where 



tusso"/' -Itivation, constant burning of fofest, scrub, and 

 mX' h"l . r ^""^5 °f-^-^ltitude of domestic animals have 

 tW t;^^''''^^ r: I^P^^^^ conditions, which approximate to 

 those of Europe." And farther on in the same piper he says, 

 T3lan'ts".'f.f "h"' vegetation is still virgin, and the introduced 

 plants altogether absent, where grazing animals have no access, 

 and where fires have never been." 



Bolle regards the Canarian endemic flora as "everlasting" and 

 indestructible," and writing of the same flora Christ views the 



intrideTs""''' ^' """ "' ^''''"''' °^ '^' "^'^''^ P^^"*" ^"^ ^^^^^^^ 

 There are very few cases of rapid spread of introductions that 

 cannot be accounted for by changed conditions, and in many of 

 these It IS probable, as in the cases of Elodsa or Eichhornia 

 water-hyacinth) in the water, or cacti or Cynara (cardoon) on 

 the land, that they have proved suitable to joining a plant 

 society which as yet was incomplete (open) and allowed room 

 for newcomers The few cases remaining, that are quite in- 

 capable of explanation as yet, are so very limited in number 

 that to base any argument upon them would be in a very high 

 degree dangerous. ^ 



The spread of introductions is often so rapid and striking that 

 one IS tempted to lay too much stress upon it, and to think that 

 the ongmal rate of spread of most species was something of the 

 same kind But there is no evidence to support this view, and 

 the natural rate of spread is often so slow that one may even 

 think that nothing is happening at all, and that a species has 

 reached its limit of distribution, whereas if things could be left 

 quite untouched for several centuries, one might find an ap- 

 preciable change at the end of that time. 



Summary 

 An erideavour is made to show that in the great majority of 

 cases the rapid spread of plants introduced into new countries 

 IS due to the changes of conditions that liave been made bv 

 man, and not to the fact that these plants have usually come 

 from more complex and "eflicient" floras. Introductions are 

 just as common upon continental areas as upon islands (to 



