98 Heredity and Environment 



organism may be analyzed into a number of characters which are 

 inherited as a whole and are not further divisible ; these are the 

 so-called "unit characters" (deVries). 



(b) The Principle of Dominance. — When contrasting unit 

 characters are present in the parents they do not as a rule blend 

 in the offspring, but one is dominant and usually appears fully 

 developed, while the other is recessive and temporarily drops out 

 of sight. 



(c) The Principle of Segregation. — Every individual germ cell 

 is "pure" with respect to any given unit character, even though 

 it come from an "impure" or hybrid parent. In the germ cells 

 of hybrids there is a separation of the determiners of contrasting 

 characters so that different kinds of germ cells are produced, 

 each of which is pure with regard to any given unit character. 

 This is the principle of segregation of unit characters, or of the 

 "purity" of the germ cells. Every sexually produced individual 

 is a double being, double in every cell, one-half having been de- 

 rived from the male and the other half from the female sex cell. 

 This double being, or zygote, again becomes single in the forma- 

 tion of the germ cells only once more to become double when the 

 germ cells unite in fertilization. 



II. Modifications and Extensions of Mendelian 



Principles 



It is a common experience that natural phenomena are found 

 to be more complex the more thoroughly they are investigated. 

 Nature is always greater than our theories, and with few excep- 

 tions hypotheses which were satisfactory at one stage of knowl- 

 edge have to be extended, modified or abandoned as knowledge 

 increases. This observation is well illustrated 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 phe- 

 nomena of inheritance now known it has become necessary to 

 modify or extend them in many ways. And yet the general and 



