498 VARIATION, MUTATION, AND ORIGIN OF SPECIES 



476. Survival of the fittest. A change in the characteristics 

 of a species may have no effect on its ability to contend with 

 a hostile soil or climate, with parasitic plants or destructive 

 insects or other animal foes ; but often alterations in the struc- 

 ture or the habits of a plant may give it a considerable advan- 

 tage over its unchanged neighbors. For instance, a decided in- 

 crease in hairiness would tend to protect the plant from damage 

 by long droughts, and also (in countries where snails destroy 

 much vegetation) from having its leaves eaten. Nuts with 

 harder shells would escape being destroyed when the ordinary 

 ones would be cracked and eaten by wild animals. Red berries 

 of the European holly are carried off by birds more extensively 

 than yellow ones, and thus the undigested seeds of the former 

 variety are more widely sown. 



In meadows which are mown once a year, only those plants 

 can surely reproduce themselves by seed which ripen their seeds 

 either before or after the time when the grass is cut. Individ- 

 uals which can do this stand a vastly greater chance of perpetu- 

 ating themselves than do those which are cut down just before 

 their seeds have matured. For this reason certain kinds of 

 meadow-frequenting plants l have developed early-blooming and 

 late-blooming forms, which would probably never have become 

 abundant in regions where the grass was not mown. 



Whatever the nature of the advantages given to one form or 

 set of forms over another in the competition which always goes 

 on under natural conditions, it results in what is sometimes 

 called survival of the fittest, and sometimes natural selection. 



477. Have species arisen by variation or by mutation ? The 

 theory that species (and later genera and higher groups) arise 

 by slow degrees from the operation of natural selection acting 

 on the slight variations w r hich constantly occur among animals 

 and plants was first fully set forth by Charles Darwin in 1858. 2 



1 Species of Gentiana, Euphrasia, and Ehinanthus. 



2 Darwin's paper on this subject was the result of over twenty years of 

 study, and was read by him to accompany a paper containing similar views 

 which had been sent from the East Indies by Alfred Russel Wallace. 



