Page:Popular Science Monthly Volume 45.djvu/623

Rh 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 during 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 latter 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 berberidaceous herb to have acquired the shrubbiness and other characteristics which distinguish barberries 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 pinnate-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 place from a sessile, simple-bladed one. From such trichotomy 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, moreover, for the curious circumstance that in these leaves there is, in addition to the articulation between leaflet and rhachis, a transverse 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 leaflet 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 concerned 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 culminated in the Glacial period, these pinnate-leaved species were exterminated 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