ESTABLISHMENT OF NEW SPECIES 205 



to escape certain theoretical difficulties which have 

 long been prominent in the minds of students of 

 evolution. Many evolutionists were accustomed to 

 argue that a new form suddenly arising in the midst of 

 an old-established species could not give rise to a new 

 and permanent variety or elementary species, because 

 it would immediately be ' swamped ' by intercrossing 

 with the parent species from which it was derived. 

 If, however, the character distinguishing the new 

 type is allelomorphic to the corresponding character, 

 or absence of a character, shown by the parent form, 

 this difficulty disappears. For suppose as an extieme 

 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 charactei 

 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 matings 

 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 



