THE STRUGGLE FOR EXISTENCE. 211 
on all changes in structure or habits which may enable plants 
to resist their living enemies or to live amid partially ad- 
verse 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 com- 
petition 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, their likenesses to each other, 
and their differences from each other to the operation of the 
struggle for existence, together with the effort to respond to 
changes in the conditions by which they are surrounded. How 
the struggle for existence has brought about such far-reaching 
results will be briefly indicated in the next section. 
255. 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 often decide which plant or 
plants of any particular kind shall lve and which ones shall 
die out. In every grove of oaks there are some with sweeter 
and others with more bitter acorns. Our shellbark hickory 
bears nuts whose shell is easily cracked by hogs, while 
another protects its seeds by a shell so hard that it is cracked 
only by a pretty heavy blow. In case of all such differences, 
there is a strong tendency to have the less eatable fruit or 
seed preserved and allowed to grow, while the more eatable 
varieties will be destroyed. Some individuals of the European 
holly produce bright red berries, while others produce com- 
paratively inconspicuous yellow ones. It has been found that 
the red berries are much more promptly carried off by birds, 
and the seeds therefore much more widely distributed, than 
the yellow ones are. The result of this kind of advantage, in 
