Variation and Heredity 277 



but at least one new whole vertebra was added at a time ; 

 and we know several cases in which the number of vertebrae 

 in the neck has suddenly been increased by the addition 

 of one more than normal, and the new vertebra is perfectly 

 formed from the first. 



In cases of this sort we can easily understand that the 

 inheritance must be either of one kind or the other, since 

 intermediate conditions are impossible, when it comes to 

 the question of one or not one ; but if one individual had 

 one and another six vertebras, then it would be theoretically 

 possible for the hybrid to have three. 



This brings us to a question that should have been spoken 

 of before in regard to the inheritance of discontinuous varia- 

 tion. It sometimes occurs that a variation, which appears 

 in other respects to be discontinuous, is inherited in a blended 

 form. Thus the two kinds of variation may not always be 

 so sharply separated as one might be led to believe. There 

 may be two different kinds of discontinuous variation in re- 

 spect to inheritance, or there may be variations that are only 

 to a greater or a less extent inherited discontinuously ; and 

 it seems not improbable that both kinds occur. 



This diversion may not appear to have brought us any 

 nearer to the solution of the difficulty that Darwin's state- 

 ment has emphasized, except in so far as it may show that 

 the lines are not so sharply drawn as may have seemed to be 

 the case. The solution of the difficulty is, I believe, as 

 follows : — 



The discontinuity referred to by Darwin relates to cases in 

 which only a single step {or mutation) has been taken, and it 

 is a question of inheritance of one or not one. If, however, six 

 successive steps should be taken in the same direction, then 

 when such a form is crossed with the original form, the hybrid 

 may inherit only three of the steps and stand exactly midway 

 between the parent forms ; or it may inherit four, or five, or 

 three, or two steps and stand correspondingly nearer to the one 



