CHAPTER IX 

 HEREDITY AND SEX 



THE chief function of both plants and animals is to 

 live and reproduce. In many wild forms the powers of 

 reproduction are little short of marvelous. A single 

 plant of purslane may produce a million seeds. Man is 

 less productive than most other mammals, but masses of 

 population have been known to double in twenty-five 

 years. At this rate in 1000 years there would not be 

 standing room on the earth for his children. 



The natural increase of plants and animals is not real- 

 ized because of unfavorable conditions. The number 

 of animals that can exist on a given area is limited. If 

 too many are born, some must inevitably die. Others 

 are destroyed by enemies, while still others are poisoned 

 by substances which accumulate within their own bodies. 



The kinds of reproduction have already been men- 

 tioned under the general subject. The simplest form of 

 reproduction is by cell division. This method of repro- 

 duction is chiefly found in unicellular organisms like the 

 amoeba and paramcecium. The most common method 

 of reproduction is by eggs. Egg production is almost 

 universal among both plants and animals. But the egg 

 generally is inert and incapable of development into a 

 new individual until it has been fertilized. Herein lies 

 the apparent reason for differentiation into male and 

 female sexes. 



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