36 PROBLET^IS OF FERTILIZATION 



are not merely senescent, if we take this view of their 

 differentiation, but they possess properties with re- 

 spect to one another that no other senescent cells 

 possess with reference either to one another or to the 

 gametes. 



The second most general aspect of fertilization con- 

 cerns the union in one individual of materials derived 

 from two, and the consequent combination of certain 

 characters of each of the parents in the zygote. The 

 entire process of biparental inheritance depends on the 

 preparation of the materials prior to union, the nature 

 of the union itself, and the subsequent redistribution of 

 such materials in development. Since fertihzation deals 

 only with the nature of the union, we shall not be con- 

 cerned with processes of inheritance. It will therefore 

 sufhce to point out here the most general impHcations 

 of this aspect of the subject. 



The union is undoubtedly the chief source of the 

 heritable variations that we can directly observe. In 

 a freely interbreeding population the germ plasms of 

 no two individuals are precisely alike, and the differences 

 often extend to numerous factors. The combinations 

 and recombinations that occur as a result of fertihzation 

 in successive generations produce, therefore, the greatest 

 possible amount of diversity of which the population is 

 capable on the basis of the organization of the germ 

 plasm, assuming equal chances and viability of all 

 crosses. Material is thus offered for natural selection 

 to work on. But beyond this there is a relation between 

 vigor of offspring and zygotic combinations, in very 

 many cases at least, in favor of heterozygous as compared 

 with homozygous matings. Certainly not all com- 



