STRUGGLE FOR EXISTENCE 521 



tected by hairs, like the Mullein, can thrive in the open on dry 

 hillsides, while there are other plants that can grow only in the 

 shade or moist ravines. The polar bear is adjusted to live 

 where the climate is cold, while the elephant is adjusted to a 

 tropical climate. Colored people can live better than white 

 people under the tropical sun because of the black pigment in 

 their skin. There are countless ways in which plants and ani- 

 mals show adjustments to particular kinds of environment, and 

 according to Darwin these adjustments are largely the results 

 of natural selection. 



Struggle for Existence. — That there is an immense struggle 

 for existence in which vast numbers of both plants and animals 

 perish is easily demonstrated. The number of seeds produced 

 by a plant and the number of offspring that mature are com- 

 monly very different. For example, one plant of the Russian 

 Thistle, one of the common weeds, produces from 20,000 to 

 200,000 seeds. Taking 25,000 seeds to a plant as a moderate 

 estimate, the offspring of one plant would number 15,625,000,- 

 000,000 in the third generation, if all the seeds grew. Allowing 

 one square foot per plant, the plants of the third generation 

 would cover more than 500,000 square miles. At this rate of 

 multiplication, there would be no room in the United States 

 for anything else in a few years. But the number of plants that 

 develop is exceedingly small in comparison with the number of 

 seeds produced. Many of the seeds are destroyed before they 

 germinate, but many plants start that do not complete their 

 development. Many are killed by insects, and many are 

 crowded out by more vigorous individuals. On an area three 

 feet long and two feet wide which had been dug and cleared so 

 that the seedlings were not choked by other plants, Darwin 

 found that out of a total of 357 seedHngs no less than 295 were 

 destroyed by slugs and insects. He also measured off a small 

 area of turf which had long been mown and allowed the plants 

 to grow. Out of the 20 species of plants growing on this small 

 plot of turf nine species, some of which were fully grown, were 

 crowded out by the more vigorous species and perished. The 

 same process of ehmination is going on among animals. Even 

 in case of the elephant, which is a very slow breeder, Darwin 

 showed that the progeny of a single pair would number 

 19,000,000 in less than 800 years, if all survived. But since the 



