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 1874. 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 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 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 
