604 THE POPULAR SCIENCE MONTHLY. 



the glacier and so take up their home in the localities we find 

 them to-day. That such a form as Berberis trifoliata, which re- 

 tains 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 primi- 

 tive trifoliolate barberry were in all probability enabled to hold 

 their own much longer against the encroaching cold, by develop- 

 ing 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 differ- 

 entiation 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 ele- 

 ments 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 trifolio- 

 late leaves are ill suited for such an arrangement the lateral leaf- 

 lets 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 bar- 

 berry 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 mi- 

 grations enabled them to continue living under conditions of cli- 

 mate 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 



