G. R. Wieland — Origin of Dicotyls. 459 



is justly that of Riitimeyer and Scribner into an adequately 

 stated general polar theory of life. 



There is therefore not only negative evidence for the Polar 

 areas as the early home of the dicotyls, but every philosophic 

 consideration tends to emphasize this as the best hypothesis. 

 It would be mere temerity to assert that a great -majority of 

 species originated in the polar areas, or that races have been 

 mainly first established there. But prepotency of Arctic cli- 

 matic factors in evolution is a philosophic conception which 

 simply cannot be ignored. In fact, there can be no question 

 that in nearly all past discussion and study of distribution the 

 actual details in the distribution of ancient life have often been 

 obscured and the adequate development of the boreal theory 

 much retarded by a form of statement amounting to an asser- 

 tion that " Arctic land bridges " were areas in which new forms 

 did not originate as freely as elsewhere; notwithstanding it 

 slowly becomes clearer that zonal distribution has existed far 

 back in time. The distribution of the Glossopteris flora first 

 suggests it, and one of the most striking facts of similar import 

 brought to light in recent years has been my own discovery of 

 the persistence of well marked Cordaites in the Oaxacan Lias. 



The dicotyls present such a varied structural facies that it 

 must be quite gratuitous to conceive them as being of mono- 

 phyletic, or in any sense restricted origin. It is much better 

 to hypothesize them as originating simultaneously in various 

 related groups already well established in the polar areas, when 

 climatic and geographic conditions were sharply varied, and 

 possibly just about the period when the very first differential 

 changes leading toward the frigid polar climates of Tertiary 

 time set in. 



The subsequent radiations of these early dicotyls over the 

 globe we might even without fossil evidence conceive to have 

 taken place with the greatest rapidity, long stretches of the 

 xerophyllous evergreen cycadophytan vegetation being replaced 

 by dicotyls with every succeeding season. All the causes for 

 such a quick movement cannot be discerned, but the mode 

 may. It is easy to see that as compared with the dicotyls the 

 unchanged cycadophytans probably were over-tenacious forms 

 holding their evergreen foliage and place in the plant com- 

 munity too stubbornly, while maturing their flowers too slowly. 

 Whereas the dicotyls, with their great possibilities of seed pro- 

 duction and continuous summer growth of succulent leaves, 

 include a vast number of forms which are quick to spread and 

 grow in every situation, and quick to die. Thus does it seem 

 that the dicotyls early proved the more potential. Able to 

 live not alone in the swamp but to form and hold on the dry 

 hillside with every fall of leaves a rich warm humus favoring 



