THE ORIGIN OF SPECIES 273 



Struggle for existence. Every plant is able to ripen many 

 more seeds than are needed to reproduce the original specimen. 

 In some annuals nearly a million seeds may be produced, and 

 the perennials often do as well and continue to do so for many 

 years in succession. But the earth is already so densely pop- 

 ulated that there is no chance for all the plants from these 

 seeds to come to maturity, even if they escape the multitude 

 of dangers that attend them on every side. Many of the seeds 

 fall in places where germination is impossible, on rocks, in 

 roads and streets, in streams and ponds, and others are eaten 

 by birds, insects, and other animals. Those that germinate are 

 subject to plant diseases and the attacks of insects, late frosts 

 may cut them down, the hot sun may burn them up, and 

 drought may cause their death. Many spring up in the shade 

 of other plants or in uncongenial soil and die for want of food, 

 while the crowd of other plants seeking the same advantages 

 with regard to light and food materials is so great that only 

 the exceptional individual is able to survive. Thus there is a 

 very real and constant struggle going on, a struggle of species 

 with species, of individual with individual, and all with the 

 untoward forces of nature. As applied to plants and animals 

 this is called the struggle for existence, and it results in the 

 survival of the fittest. Here we discover one reason for the 

 numerous seeds produced by plants. The greater the mimber 

 produced, the greater the chance that at least a few will 

 survive to replace the original form. 



Natural selection. The plants that survive all the vicissi- 

 tudes of nature and finally come to maturity are, as we have 

 seen, those that in the long run are best fitted to survive. A 

 slightly larger amount of food in the seed may have given 

 them a start over weaker plants in the vicinity, a more rapidly 

 growing root may have brought them into contact with mois- 

 ture sooner, the ability to get along with less light may have 

 enabled them to survive, or any one of a hundred other things 



