208 



EVOLUTION, GENETICS, AND EUGENICS 



twenty chromosomes — less than half that possessed by man — there 

 would be (2) 10 , or 1,024 different gametes in each sex. And since each 

 kind of egg is as likely to be fertilized by one kind of sperm as another, 

 there might be produced (1024) 2 , or 1,048,576 different zygotes. 

 Hence, meiosis and fertilization together constitute an extremely effec- 



Fig. 50. — Diagram to illustrate fertilization, d% male pronucleus; 9 , female 

 pronucleus; observe that the chromosomes of maternal and paternal origin re- 

 spectively do not fuse. {From Guyer.) 



tive mechanism for increasing diversity. This is the chief role played 

 by sex in evolution. 



This abbreviated account of biological processes is all that is needed 

 for our purpose and should, we believe, enable the student to follow 

 intelligently the accounts of heredity, variation, selection, and isola- 

 tion that are to follow. A more detailed account of the processes of 

 mitosis, meiosis, and fertilization may be found in chapter xliv, "The 

 Mechanism of Mendelian Heredity." 



