REGIONAL DISTRIBUTION 501 



pieces of rootstocks, or even as entire plants washed down 

 by the stream. Such immigration among plants is in accord- 

 ance with the principle known as following the line of least 

 resistance; that is, immigration occurs whenever and wher- 

 ever a more densely populated area is in contact with one 

 less densely populated, or when a set of plants of superior 

 qualifications to occupy a given territory comes into contact 

 with a set less well qualified. 



454. Plants able to meet the conditions of their environment ; 

 exceptions. A little observation and reading is enough to con- 

 vince the student that the plants of a region usually meet 

 fairly well the requirements of its soil and climate. That is 

 to say, wild plants occur where they do, largely because of 

 the fact that when they migrated into the area which they 

 now occupy they were well equipped to contend with other 

 plants and hold their own in this environment; or because, 

 after immigrating, they developed characteristics which en- 

 abled them to succeed. If a considerable piece of land with 

 its flora could be transplanted bodily from the arctic regions 

 to the most fertile part of the tropics, its vegetation would be 

 promptly destroyed by the new climatic conditions or by the 

 luxuriant growth which would invade it from every side. On 

 the other hand, if a piece of ground covered with tropical 

 vegetation were carried into arctic winter conditions, its plant 

 life would perish of cold in a few hours. 



Exceptions to the rule that plants are found growing in the 

 places that suit them fairly well are not uncommon. A little 

 has already been said (Sect. 434) about the extraordinary way 

 in which some plants multiply on first being brought into terri- 

 tory new to them. Some of the most noteworthy instances 

 of displacement of native plants by foreign species have oc- 

 curred in New Zealand. There many large, robust plants, 

 some of them very spiny and growing in masses impenetrable 

 by cattle, have been replaced by European grasses and clovers. 

 The sweetbrier destroys pastures and also drives out native 

 shrubs; and the black locust, with its rapid growth and 



