222 MENDELISM 



or absence of a character, shown by the parent form 

 this difficulty disappears. For suppose as an extreme 

 case that the new type arises as a single individual 

 only, which is therefore compelled to mate with a 

 member of the original species. If the new character 

 is recessive it will disappear in the immediate offspring 

 of this cross. But half the germ-cells produced by 

 the cross-bred form will bear the new character pure 

 and undiluted. If any of these cross-breds mate to- 

 gether the new type will appear in a quarter of their 

 offspring. Even if all of them mate with members of 

 the original type, half the offspring of such mat ings 

 will be heterozygous, and sooner or later the hetero- 

 zygotes will be sure to mate with one another, and 

 give rise once more to the novel type of individuals. 

 If the new form has any structural or other advantage 

 over the old species, the former will tend to survive at 

 the expense of the parent type, and it may survive 

 if it is only equally well fitted for the battle of life. In 

 the case of dominance of the new form the same 

 process will take place, only it will be apparently more 

 rapid in the early stages because the cross-breds will 

 themselves exhibit the new character. In this case, even 

 if the new type has a very marked advantage over the 

 parent form, the process of completely supplanting the 

 latter will be delayed, because the old type of character 

 can survive concealed in heterozygote individuals. 



Let us pause for a moment to sum up the novel ideas 

 which have so far been presented in this and the 

 preceding chapter. 



We found in the first place that from the point 



