76 University of California Publications in Agricutlural Sciences [Vol. 2 



emphasize the fact that hindsii is distinct from calif ornica even 

 in the young seedling stage, a typical seedling of californica 

 and one of hindsii are shown in plate 21, figures 5 and 6. The 

 differences between the nuts of the two species are clearly shown. 

 The mature trees of the two species are also strikingly distinct, 

 calif ornica being always low and shrub-like in habit while hindsii 

 is tall and arboreous in form. 



These parallel mutations in two distinct species may appear 

 as degressive rather than regressive variations or, in other words, 

 as cases of reversion to a common ancestral form. If both quer- 

 cina and quercinifolia resulted from a change in one genetic 

 factor, as seems likely, and both represent a common ancestral 

 form, then it would follow that both calif ornica and hindsii 

 sprang fully formed from their common ancestor by mutation. 

 Yet both quercina and quercinifolia show reduction in morpho- 

 logical characters and quercina individuals exhibit low fertility. 

 These symptoms would indicate that the mutation is regressive 

 rather than degressive. However, the fact that any walnut varie- 

 ties originate by mutation is of significance for the student of 

 evolution, because the Juglandacae are generally considered as 

 one of the older and more stable groups of angiosperms. They 

 are not supposed to be undergoing changes similar to changes 

 that give rise to new types in the younger, less stable groups. 

 That the origin of these two unique walnuts, or of quercina at 

 least, cannot be explained on the basis of hybridization is now 

 fully proved. Evidently quercina sprang from calif ornica rather 

 than calif ornica from quercina, and it arose as a result of a 

 change in a single genetic factor, i.e., of mutation in the strict 

 sense. 



Transmitted May 31, 1916. 



