CHAPTER III 



THE MULTIPLICATION OF ANIMALS AND SEX 



28. All life from life. On the performance of the func- 

 tion of reproduction or multiplication depends the exist- 

 ence or perpetuation of the species. Although an animal 

 may take food and perform all the functions necessary to 

 its own life, it does not fulfill the demands of successful 

 existence unless it reproduces itself. Some individuals of 

 every species must produce offspring or the species becomes 

 extinct. We have seen in our study of the simple animals 

 that the function of reproduction is the first function to 

 become differentiated in the ascent from simplest animals 

 to complex animals. The first division of labor among the 

 cells composing the bodies of the slightly complex animals 

 and the first structural differences among the cells are 

 connected with the performance of the function of repro- 

 duction or multiplication. 



We are so familiar with the fact that a new kitten 

 comes into the world only through being born, as the off- 

 spring of parents of its kind, that we shall likely not appre- 

 ciate at first the full significance of the statement that all 

 life comes from life ; that all organisms are produced by 

 other organisms. Nor shall we at first appreciate the im- 

 portance of the statement. This is a generalization of 

 modern times. It has always been easy to see that cats 

 and horses and chickens and the other animals we famil- 

 iarly know give birth to young or new animals of their 

 own kind ; or, put conversely, that young or new cats and 

 horses and chickens come into existence only as the off- 

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