EVOLUTION OF ANIMALS AND PLANTS 569 



develop beyond the seedling stage, for there is not space and 

 food enough. The vast majority will be eaten by animals, de- 

 stroyed by unfavorable weather, or killed by crowding such 

 as one can see in any place where plant seedlings are numerous. 



Turn where we will among animals and plants, the facts 

 are essentially as stated for the oyster and weeds : the total 

 number of young organisms is so enormous that there must 

 be a struggle for existence and destruction of the vast majority. 

 This leads to many complicated relations between organisms. 

 Food must be' obtained, and this means competition between 

 those which require the same kind of food, between carnivora 

 and their prey, and between animals and plants. Also, there 

 is a constant struggle with the environment (e.g., crowded 

 plants struggling for light and water; and various animals 

 against adverse climatic conditions). These are only sug- 

 gestions of some ways in which nature subjects organisms 

 to struggle in an intense competition, which is caused chiefly 

 by the fact that all forms of life reproduce more rapidly than 

 necessary to maintain a constant number of individuals of 

 each species. 



There are reasons for believing that the struggle for exist- 

 ence does not result in indiscriminate destruction of indi- 

 viduals. Some are better fitted than others to withstand the 

 adverse conditions. For example, plant twenty sunflower 

 seeds in a flower-pot, and gradually one or two of the seed- 

 lings will grow above the others. They are best fitted and 

 therefore win in the struggle for existence. The same thing 

 is constantly happening in all species of organisms. Those 

 which survive and reproduce are the fittest. The weakest 

 and least adapted soon perish. Thousands of forms of 

 animals and plants which once lived on the earth have dis- 

 appeared because they were not properly fitted for the struggle 

 with changed environment and improved competitors. The 

 effect of all this upon origin of new species will be discussed 

 in the next paragraph. 



