222 MENDELISM 
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 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 
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 sup- 
planting 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 
