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 xcith 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 live 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 tlie 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 



