THE STRUGGLE FOR EXISTENCE 395 



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. One shellbark hickory bears nuts 

 whose shell is easily cracked by hogs, while another pro- 

 tects 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 eat- 

 able varieties will be destroyed. Some individuals of the 

 European holly produce bright red berries, while others 

 produce comparatively 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 any of its countless forms, is 

 sometimes called survival of the fittest, and sometimes 

 natural selection. The latter name means only that the 

 outcome of the process just described, as it goes on in 

 nature, is much the same as that of the gardener's selection, 

 when, by picking out year by year the earliest ripening 

 peas or certain kinds of the oddest-colored chrysanthe- 

 mums, he obtains permanent new varieties. Natural 

 agencies, acting on an enormous scale through many 

 ages, may well be supposed to have brought about the 

 perpetuation of millions of such variations as are known 

 to be of constant occurrence among plants, wild as well 

 as cultivated. 



