394 FOUNDATIONS OF BOTANY 



In our own country a noteworthy case of the kind has 

 occurred so very recently that it is of especial interest 

 to American botanists. The so-called Russian thistle 

 (Fig. 268), which is merely a variety of the saltwort, so 

 common along the Atlantic coast, was first introduced into 

 South Dakota in flaxseed brought from Russia and planted 

 in 1873 or 18 74. In twenty years from that time the plant 

 had become one of the most formidable weeds known, over 

 an area of about twenty-five thousand square miles. 



464. Importance of Adapt! veness in Plants. It may be 

 inferred from the preceding sections that a premium is set 

 on all changes in structure or habits which may enable 

 plants to resist their living enemies or to live amid partially 

 adverse surroundings of soil or climate. It would take a 

 volume to state, even in a very simple way, the conclusions 

 which naturalists have drawn from this fact of a savage 

 competition going on among living things, and it will be 

 enough to say here that the existing kinds of plants to a 

 great degree owe their structure and habits to the operation 

 of the struggle for existence, this term including the effort to 

 respond to changes in the conditions by which they are sur- 

 rounded. How the struggle for existence has brought 

 about such far-reaching results will be briefly indicated in 

 the next section. 



465. Survival of the Fittest. When frost, drought, 

 blights, or other agencies kill most of the plants in any 

 portion of the country, it is often the case that many of 

 the plants which escape do so because they can stand more 

 hardship than the ones which die. In this way delicate 

 individuals are weeded out and those which are more 

 robust survive. But other qualities besides mere toughness 



