the glacier and so take up their home in the localities we find them to-day. That such a form as Berberis trifoliata, which retains so fully the primitive characteristics, should remain in Mexico, seems to find a sufficient explanation in the fact that the climate of this region resembles most closely that of its supposed northern home in preglacial times; or, in other words, we may look upon the persistence of the original form as connected with the continuation of similar climatic conditions during the life of the species from the time when the genus first appeared.
While the ancestors of our modern mahonias were seeking an asylum in lower latitudes, certain other descendants of the primitive trifoliolate barberry were in all probability enabled to hold their own much longer against the encroaching cold, by developing those adaptations to extremes of temperature which make the various forms of euberberis so well suited to their present home.
We have already seen the advantages which come with differentiation of the branch system when plants are to be subjected to the storms of a severe winter. Such differentiation, however, means not only a more efficient disposition of the mechanical elements in the stem part of the plant, but it involves a closer and closer crowding of the leaves on the shorter branches until the limit of crowding is reached in the rosette. Obviously trifoliolate leaves are ill suited for such an arrangement—the lateral leaflets would be so much in the way. The causes which bring about the reduction and final disappearance of parts that have become useless or harmful to a species could not fail, therefore, to affect these leaflets until the present unifoliolate condition was reached. Moreover, in the absence of lateral leaflets there would be less need for an elongated leafstalk, and we should expect, therefore, just such an abbreviation of this organ as we actually find in a large share of the species of euberberis. We have already noticed how this enables Berberis vulgaris to turn its petioles to good account, by keeping them as protective bark scales long after the leaf blades have fallen.
It is in harmony with our conclusion that the ancestral barberry was a holly-like plant, whose descendants became modified under the influence of gradual refrigeration, to suppose that the earlier forms of euberberis were evergreen. So far as their migrations enabled them to continue living under conditions of climate favorable to the retention of leaves throughout the year, this habit might be expected to be present. This we find is the case with species in central Asia and in the mountainous and temperate parts of South America. Even in a region of much snow and ice no serious disadvantages need be feared, provided the plant does not extend its branches far above the ground. This will doubtless explain the presence of the evergreen mahonia
