variation may have occurred many times in 

 rats, as it has in so many other mammals, but 

 failed to become visible simply because it has 

 the same potency as in most mammals, but is 

 subject to the same physiological limitations 

 as in mice, so that it cannot exist in a homo- 

 zygous state. In that case the only evidence 

 of its existence in a race would lie in a 

 slightly diminished fecundity under inbreeding, 

 as is found to be the case in yellow mice. 



Such sharply contrasted variations in the 

 potency of characters as we have been discuss- 

 ing are evidently of prime importance in evo- 

 lution, making all the difference between a 

 dominant and a recessive condition of a char- 

 acter, or between the occurrence and the per- 

 manent suppression of a particular variation. 

 The character which is potent enough to show 

 itself in a single dose will behave as a domi- 

 nant character in crosses. We might call it 

 nnipotent. That which must be present in a 

 double dose to produce a visible result will 

 behave as a recessive character in crosses. We 

 might call it semi-potent. It is not impossible 

 that the same character may as regards domi- 



97 



