302 EVOLUTION, GENETICS, AND EUGENICS 



superior individual, one better able to meet the struggle for existence 

 than any individual of either uniting strain. It is by taking advantage 

 of the mechanism of sexual reproduction that the animal- and plant- 

 breeder has been able vastly to improve his stocks. Deliberate experi- 

 ments have successfully been performed with the aim of uniting the 

 favorable characteristics of several species or varieties and of eliminat- 

 ing the unfavorable characteristics. It was in this way that the 

 famous Chanticleer breed of fowl was created. 



The processes of maturation and fertilization in the germ cells in- 

 volve a sort of shuffling and dealing of hereditary units. The shuffling 

 mechanism takes place during the period prior to the maturation 

 divisions of the germ cells, and involves the pairing of homologous 

 chromosomes, crossing-over, and other phenomena that are to be 

 explained later. The dealing process is involved in the formation of 

 gametes and in the union of these gametes to form zygotes. 



In the next chapter appears a somewhat detailed account of the" 

 whole machine of gametic reproduction. Some of the facts brought 

 out in that chapter will not find their fuU significance until the reader 

 becomes familiar with the hereditary behavior of individual differences 

 as brought out in the Mendelian laws. In spite of the fact that some 

 teachers of genetics prefer to postpone all consideration of the mecha- 

 nism of germ-cell reproduction until the studies of Mendelian regulari- 

 ties require an explanation in terms of germ cells and chromosomes, it 

 has been our experience that at least a preliminary knowledge of the 

 cellular basis of heredity greatly facilitates the understanding of 

 Mendelian heredit3^ 



