8] MODIFICATION AND DISTRIBUTIONS OF CROPS 221 



agg.). But that is merely on territory where the native vegetation 

 has been disturbed or destroyed : if this native vegetation is left 

 alone to develop naturally, the aliens, however rank and aggressive 

 they may once have been, will in most instances disappear within a 

 very few years. Likewise have the plants which used to be trans- 

 ported in ships' ballast largely disappeared, since the discontinuation 

 of the dumping of such ballast, from the stations in which they 

 formerly grew as aliens. The discontinuation of a road or railway- 

 line is apt to have a similar effect, and even those aliens which have 

 managed to spread from the immediate vicinity of the travelled track 

 usually disappear when Man's influence is removed and the sur- 

 rounding vegetation comes back into its own. 



All this does not mean that Man's influence in changing the dis- 

 tribution of plants is other than enormous, but rather that it is in 

 many instances merely temporary, as the plants involved have 

 become only incompletely naturalized and certainly not lastingly 

 acclimatized. It should also be remembered that, with Man's 

 increasing mobility, for every disappearance of a plant from an area 

 there is probably on the average at least one new introduction else- 

 where, though in this connection particular plants tend to have their 

 ups and downs. Even in those parts of the world, such as New 

 Zealand and Hawaii, where the native plants have been largely ousted 

 over considerable tracts by adventive aliens, this has happened only 

 following disturbance of the native plant communities as well as 

 importation of the aliens by Man or his domestic animals. There 

 it is widely contended that removal of Man's influence would lead 

 to a reversal of the situation through return of the natives whose 

 stronger competition would ultimately oust the alien colonists. 

 This matter of strength of competition is one of the most important 

 in the life of organisms, and is often the key to the present-day 

 distribution of plants as well as to their potential ranges. Crop 

 plants, sheltered and pampered as they are (and have usually long 

 been accustomed to being), are notoriously weak in competition, 

 and consequently rarely to be found in a truly naturalized state. 



The adaptations of plants to particular habitat conditions are 

 varied and sometimes so precise as to remain unnoticed, yet suflicient 

 to prevent the leading of an independent life. Often a mere slight 

 change in environmental conditions will threaten the very existence of 

 a species. For example, a Mexican species of Birthwort {Aristolochia) 

 when transplanted to Java flowered abundantly but failed to bear 

 fruit — not because of any lack of pollinators but because the climate 



