148 THE THEORY OF MUTATION 



from the parental type, yet their first appearance has 

 nothing to do with normal or continuous variability. 



6. The mutations take place indefinitely, showing 

 no special tendency in any particular direction. 



7. The tendency to mutate recurs periodically. But, 

 as was previously stated, there is no direct evidence 

 of this last supposition. 



In addition to what has already been said with 

 reference to the method of origin of garden varieties in 

 general, de Vries has described a number of special 

 phenomena regarding the behaviour of garden varieties 

 of plants, some of which are of considerable interest. 

 Taken together, the facts substantiate to a great extent 

 the view that selection does not of itself lead to the 

 production of specific characters. But de Vries also 

 introduces certain new conceptions which require to 

 be briefly described on account of their great general 

 interest to practical breeders and gardeners. They 

 consist in the idea of races existing intermediate be- 

 tween a species and a complete variety or sub-type of 

 it. Such between-races are of two kinds, of which it is 

 unusual to find both in the case of the same species ; 

 moreover, either of them may occur even when the 

 complete variety is quite unknown. In the case of a 

 half -race a small percentage only of seedlings is found 

 to produce plants which show the racial character, the 

 remainder being of the original specific type ; and even 

 if the racial type is selected for several generations, the 

 percentage of plants of this type which is produced 

 does not notably increase. A mid-race, on the other 

 hand, can readily be improved by selection, and when 



