stability and Real Atavism 171 



very narrowly restricted one, and includes only 

 those ancestors from which a form is known to 

 have been derived. But such evidence is of 

 course historic. If a variety has been observed 

 to spring from a definite species, and if the cir- 

 cumstances have been sufficiently ascertained 

 not to leave the slightest doubt as to its pure 

 origin, and if moreover all the evidence has 

 been duly recorded, we may say that the origin 

 of the variety is historically known. In most 

 cases we must be content with the testimony, 

 given somewhat later, and recorded after the 

 new variety had the opportunity of showing 

 its greater merits. 



If it now happens that such a variety of re- 

 corded origin should occasionally revert to its 

 parent-species, we have all we can wish for, in 

 the way of a thoroughly proved case of atavism. 

 But such instances are very rare, as the birth 

 of most varieties has only been very imper- 

 fectly controlled. 



Next to this comes the systematic relation of 

 a variety to its species. The historic origin of 

 the variety may be obscure, or may simply be 

 forgotten. But the distinguishing marks are 

 of the order described in our last lecture, either 

 in the positive or in the negative direction, and 

 on this ground the rarer form is considered to 

 be a variety of the more wide-spread one. If 



