NORMAL FERTILIZATION 203 



good is to suppose that the same rule applies to cases 

 of normal fertilization as to hybrid fertilizations. We 

 should then picture the former process as taking 

 place in somewhat the following way. Every visible 

 character of the individual which can be separately 

 distinguished, and which on cross-breeding would be 

 inherited on ordinary Mendelian lines, must be repre- 

 sented in the gametes by a definite factor of some kind, 

 possibly by a definite substance or combination of sub- 

 stances. The pair of parental factors for a particular 

 character would combine on fertilization, and at the 

 formation of the gametes in the offspring its members 

 would separate as perfectly definite entities, to re- 

 combine when these gametes meet once more with their 

 corresponding mates. Such a definite segregation of 

 characters taking place within a pure strain would be 

 very difficult of absolute demonstration, but it is hard 

 to avoid the conclusion that this is a true deduction 

 from the facts observed when cross-breeding takes 

 place. Such a segregation would formerly have been 

 thought a very small assumption in comparison with 

 that of the segregation of pairs of allelomorphs of 

 which no trace is externally visible, and yet the latter 

 assumption has now been shown to be perfectly well 

 established. 



This idea of unit characters, capable of being inherited 

 independently of one another, is one of the most 

 important conceptions which has ever been introduced 

 into the science of biology, and the introduction of it 

 has followed as the direct result of Mendel's work. It 

 is a conception which has led to a complete change in 



