76 REPRODUCTION AND LIFE HISTORY. 



If two parents differ in two crisply defined characters, the fertilised 

 egg-cell which they unite in producing will contain the "factors" or 

 "genes" for these two characters. In cases of Mendelian inheritance 

 these two hereditary factors do not blend, but in the course of the 

 maturation of the germ-cells of the ' ' hybrid " offspring the maternal and 

 paternal chromosomes bearing the respective factors get assorted apart, 

 so that one contingent of germ-cells contains the paternal factor or 

 gene, and the other contingent the maternal factor or gene. In sub- 

 sequent random pairing among the "hybrids" or D(R)s the Mendelian 

 proportion (iD + 2D(R) + iR) is bound to be expressed in the next 

 generation. 



Heredity may be defined as the relation of genetic 

 continuity between successive generations, and inheritance 

 as all that the organism is or has to start with in virtue 

 of this hereditary relation. Development is the expression 

 or realisation of the heritable qualities which have their 

 physical basis in the germ cells, and it presupposes an ap- 

 propriate environment of nutrition and " liberating stimuli," 

 "nurture" in the widest sense. What the organism 

 becomes is the resultant of two components, inherited 

 " nature " and external " nurture." 



