SURVIVAL OF THE FITTEST 205 



miles of territory. The newcomers were more vigorous, or 

 in some way better adapted to get on in the world, than 

 the native plants which they encountered, and so managed 

 to crowd multitudes of the latter out of existence. 



In our own country a noteworthy case of the kind has 

 occurred so recently that it is of especial interest to Ameri- 

 can botanists. The so-called Russian thistle (Fig. 144), 

 Salsola Kali, var. tenidfolia, a variety of the saltwort of 

 the Atlantic coast, was first introduced into South Dakota 

 in flaxseed brought from Russia and planted in 1873 or 

 1874. In twenty years from that time the plant had be- 

 come one of the most formidable weeds known over an 

 area of about twenty-five thousand square miles. 



253. Importance of Adaptiveness 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 hinds 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 hy ivhich they are sur- 

 rounded. How the struggle for existence has brought 

 about such far-reaching results will be briefly indicated in 

 the next section. 



254. 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 



