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 seedlings 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 elimination 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 
