568 EVOLUTION 



Among animals the effects of the struggle for existence is no 

 less pronounced than among plants. It has been estimated 

 that a single green fly would produce enough descendants in one 

 summer, if all lived and multiplied, to weigh down the population 

 of China. Likewise the descendants of a single oyster, if all 

 hved and multiplied until there were great-great-grand children, 

 would produce a heap of shells much larger than the earth. Even 

 among the largest of animals, as in the case of the elephant, 

 Darwin showed that only a few out of the many individuals 

 survive. 



Both plants and animals multiple much faster than space and 

 food supply permits and it is through the struggle for existence 

 that the number of living beings is kept in equilibrium with space 

 and food supply. 



Survival of the fittest. — According to their qualifications to 

 compete in the struggle for space, food and other necessities of 

 life, some living beings survive while others perish. Many must 

 perish in order that those that survive may have the requisite 

 amount of space, food, etc. As to which individuals survive and 

 which perish in the struggle, that depends upon the individual 

 differences or variations that count as advantages or disadvan- 

 tages in the struggle. The advantages one individual has over 

 another may be due to a difference in rate of growth, in absorbing 

 power of root system, in the rapidity of manufacturing food, in 

 ability to resist drought, cold, heat, the attacks of insects or 

 disease producing organisms, etc. In case of animals the advan- 

 tage may lie in the abihty to excel others in running, fighting, 

 enduring unfavorable weather, resisting diseases, etc. The bull 

 with the sharpest horns and strongest neck, the hog with a snout 

 best adapted to rooting, the rabbit and squirrel most skillful in 

 eluding pursuers, etc., have an advantage over others of their 

 kind not so well prepared to take care of themselves. The 

 individual best adapted to get the necessities of life and withstand 

 the attacks of enemies has the best chance to survive. In other 

 words the fittest survive and the less fit perish. The process is 

 just as much a rejection of the unfit as it is the survival of the 

 fittest, for it is through nature's rejection of the unfit that the fit 

 individuals have an opportunity to live out their life cycle and 

 perpetuate their fitness in offspring. To this sifting process in 

 nature, Herbert Spencer applied the phrase "Survival of the 



