PHENOMENA OF INHERITANCE 337 



again and again, so that there is no doubt that they constitute an im- 

 portant rule of inheritance among all organisms. 



In brief the "Mendelian Law of Alternative Inheritance" or of 

 hereditary " splitting " consists of the following principles : 



(a) The principle of unit characters. — The total heritage of an 

 organism may be analyzed into a number of characters which are in- 

 herited as a whole and are not further divisible; these are the so-called 

 "unit characters" (deVries). 



(b) The principle of dominance. — When contrasting unit char- 

 acters are present in the parents they do not as a rule blend in the off- 

 spring, 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 derived from the male and the other half from the female 

 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 

 srerm cells unite in fertilization. 



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(To be concluded) 



