BARBERRIES. 603 



spread over the whole of Asia, and invaded almost every part of 

 Europe."* Of these latter genera Berberis is surely one. 



Geologists tell us that the climate even of arctic America dur- 

 ing the Mesozoic era was as warm and equable as that of our 

 Southern States to-day. The same was true of northern Europe 

 and Asia, and there is good reason to believe that between the lat- 

 ter and America there was during Mesozoic times a continuous 

 land connection in high latitudes, or at least a chain of islands 

 uniting the two continents. Such were the conditions then under 

 which we may suppose a berberidaceons herb to have acquired 

 the shrubbiness and other characteristics which distinguish bar- 

 berries from the rest of the family. 



As the descendants of this trifoliolated, woody form multiplied 

 and spread over the vast territory open to them, the modifications 

 which arose must have progressed along two principal lines of 

 development. The first to diverge was doubtless the line of pin- 

 nate-leaved mahonias. To explain the development of such a leaf 

 from the trifoliolate ancestral form, we have only to suppose the 

 terminal leaflet to become stalked and then divided into three, just 

 as we must conceive the trifoliolate leaf to have been derived in 

 the first i^lace from a sessile, simple-bladed one. From such tri- 

 chotomy of the terminal leaflet would result a five-foliolated leaf 

 (Fig. 6) ; but let the process be repeated with successive terminal 

 leaflets a sufficient number of times, and the most highly developed 

 mahonia leaf is readily accounted for. This view accounts, more- 

 over, for the curious circumstance that in these leaves there is, in 

 addition to the articulation between leaflet and rhachis, a trans- 

 verse articulation extending across the rhachis between each pair 

 of leaflets (Fig. 6 A). For what can this be but the representative 

 of that terminal articulation which was once at the base of a leaf- 

 let since differentiated into all those parts of the leaf now lying 

 above the articulation ? 



Along with the multiplication of leaflets there appears to have 

 been a lessening of the number of leaves and some shortening of 

 the branches, which affected the lateral ones somewhat more than 

 the primary axes ; but beyond this the changes introduced con- 

 cerned only matters of small detail. The descendants of this new 

 form spread into Asia and thence into Europe, where we find some 

 remains of them in the deposits of the Tertiary. Subsequently, in 

 the course of that general lowering of temperature which culmi- 

 nated in the Glacial period, these pinnate-leaved species were ex- 

 terminated in Europe, while in Asia and America, where a more 

 southerly extension into warmer regions was possible, they were 

 able to survive and spread northward again after the retreat of 



* Nat. Hist. Review, p. 370. 



