258 HEREDITY AND ENVIRONMENT 



male sex cell. This double being, or zygote, 

 again becomes single in the formation of the 

 germ cells only once more to become double 

 when the germ cells unite in fertilization. 



II. Modifications and Extensions of Men- 

 delian Principles 



It is a common experience that natural 

 phenomena are found to be more complex the 

 more thoroughly they are investigated. Na- 

 ture is always greater than our theories, and 

 with few exceptions hypotheses which were 

 satisfactory at one stage of knowledge have to 

 be extended, modified or abandoned as knowl- 

 edge increases. This observation is well illus- 

 trated in the case of the Mendelian theory. 

 The principles proposed by Mendel were rela- 

 tively simple, but in attempting to apply them 

 to the many phenomena of inheritance now 

 known it has become necessary to modify or 

 extend them in many ways. And yet the gen- 

 eral and fundamental truth of these principles 

 has been established in a surprisingly large 

 number of cases, and they have been extended 



