PROBLEM 4. Theories about the^ Ori 



Fig. 506 A puff ball is related to the musb- 

 roovis. It cracks open aiid discharges its spores. 

 It has been calcidated that a large puff ball may 

 contain 6,000,000,000 spores, (clarke) 



Darwin's theory of natural selection. In 



the theory of natural selection Darwin 

 tried to explain by what method organ- 

 isms changed throughout the ages or 

 how new species originated. His theory 

 includes five steps. Let us examine them: 

 I. Overproduction. Aiost animals and 

 plants produce offspring at so great a 

 rate that if all lived and reproduced there 

 would not be standing room on the 

 earth for the individuals of even one 

 species. If a microscopic one-celled ani- 

 mal like Paramecium divided twice a 

 day for only a few months it would pro- 

 duce a ball of paramecia as big as the 

 earth. The total number of descendants 

 after ten generations of a fish Hke the 

 cod, which lays about 8,000,000 eggs 



gin of Kivds of Living Thiiigs 557 



every year, is staggering. It would fill 

 the oceans from shore to shore with cod- 

 fish. Darwin stressed the well-known 

 fact that there is overproduction of liv- 

 ing things, both plant and animal. See 

 Exercise 4. 



2. Struggle for existence. Since there 

 is not food enough or room enough for 

 all the offspring of plants and animals, 

 some of them must die before growing 

 up. If the number of codfish in the 

 ocean remains about the same it means 

 that only two of the 8,000,000 eggs re- 

 sult in mature codfish and 7,999,998 of 

 that family die before they can repro- 

 duce. Darwin pointed out that overpro- 

 duction leads to a struggle for existence 

 among plants and animals. Under ordi- 

 nary circumstances, the more offspring 

 produced, the keener is the struggle. 

 This struggle is not necessarily a fight 

 between codfish or between codfish and 

 other animals. For the most part, the 

 struggle for the necessities of life is a 

 quiet, steady struggle that is difficult to 

 observe. It is a struggle that goes on 

 steadily between members of the same 

 species and between different species. 

 All organisms in nature enter into the 

 struggle; on the average, only two of a 

 family survive to take the place of their 

 parents. For one reason or another some- 

 times more survive; then the species in- 

 creases in numbers for a while, but the 

 struggle goes on. 



3. Variation. Which of these many 

 organisms will survive? Darwin an- 

 swered this question by pointing out 

 that all the members of a species are 

 different in many small ways. You know 

 that this is true; you even know the 

 causes of such variations. They may be 



