part 2] ANNIVERSARY ADDRESS OP THE PRESIDENT. ciii 



degree of differentiation and the variety of the genera. It will, 

 however, be more appropriate and more convenient to consider the 

 composition of these floras when we follow the history of the 

 vegetation of the world through the closing scenes of the Palaeozoic 

 Era. Meanwhile, I shall content nryself with a brief reference to 

 the distribution of the genus Archceopteris. The type-species, 

 described by Edward Forbes in 1852 from the Upper Devonian 

 rocks of Kilkenny as Cyclopteris liibernica, was renamed by Sir 

 William Dawson Archceopteris liibernica : it is represented by 

 large, compound, fernlike fronds similar in habit to those of 

 certain recent species, and characterized by its cuneate leaflets and 

 dense clusters of sporangia. We are still in doubt as to the precise 

 systematic position of the genus ; like recent Ferns, it may have 

 possessed spores of one kind only, or, on the other hand, it may be 

 a member of the extinct group of Pteridosperms. The several 

 species are distinguished by relatively slight differences in the size 

 and degree of dissection of the leaflets. Archceopteris is recorded 

 from Upper Devonian strata in Ellesmere Land, Bear Island, 

 Canada, Pennsylvania, Berwickshire, Ireland, Belgium, Germany, 

 Russia, Australia, and elsewhere. The occurrence of Archceopteris 

 and other genera as far north as lat. 80° N. and in Bear Island, 

 where the flora seems to have been at least as rich and vigorous as 

 in the South of Ireland, is a remarkable fact that raises climatic 

 problems, of which as yet no satisfactory solution has been found. 

 It is difficult to picture these plants completing their life-histories 

 in the brief span of an Arctic summer : we talk glibly of sub- 

 tropical or even tropical conditions in far northern regions, without 

 sufficiently realizing the difficulties from the point of view of the 

 plants. Admitting the probability of the assumed existence of a 

 higher temperature than was actually required by the plants 

 discovered in Polar lands, the problem of the Arctic night and its 

 effect upon the vegetation still remains. We have much to learn 

 from experimental work about the ability of plants to endure the 

 long alternating periods of continuous illumination and compara- 

 tive darkness, and we are hardly in a position to demand as a 

 necessity either a shifting axis or a wandering crust. 



The differences between the older and the Upper Devonian 

 floras may well be connected with differences in environment, as 

 well as with the march of evolution. Before the end of the 

 Devonian Period the terrestrial vegetation had come into its own, 

 and had colonized the hisrher and drier ground in addition to the 



