214 SECTIONAL ADDRESSES. 



suggest that the relation of genera and species to climate to which we are 

 accustomed is merely a phase in the history of plants ? A plant that is 

 now confined to the tropics may at a much earlier stage of its career have 

 been able to live under other conditions. In using plants as thermometers 

 of the ages, we accept as an axiom the principle — what is now has always 

 been. Our vision is limited by what we see and beyond the horizon we 

 see only in imagination. Is it unscientific to express the opinion that we 

 may think of plants not only as organisms which have changed in form and 

 structure in the course of thousands or millions of years, but as organisms 

 which have changed also in their susceptibility to external factors ? There 

 is another point, and an obvious one ; instances are common enough of 

 species of living genera which exist under conditions sharply contrasted 

 with those characteristic of the majority of species of the same genus. 

 The Cretaceous and other plants are practically all specifically distinct 

 from their living descendants • we are not entitled to attribute to extinct 

 and recent alike the same constitutional qualities. 



I suggest that there is a tendency to rate too highly the value of extinct 

 plants as guides to climatic conditions, and I would again emphasise the 

 desirability of obtaining more definite information than is at present avail- 

 able on the effect of continuous light and continuous darkness, under 

 suitable temperatures, on plants which do not at present occur in Arctic 

 habitats. Even if the foregoing suggestions have any merit, and if we 

 have underestimated the capacity of plants to survive Arctic seasons, there 

 is still a serious obstacle to surmount before it is possible to imagine, let 

 us say, the Rhsetic vegetation of Scoresby Sound and that of Southern 

 Sweden flourishing in regions separated from one another by at least ten 

 degrees of latitude. It seems impossible to get away from the conviction 

 that there must have been in the past as there is now a marked contrast 

 between two sets of contemporary plants, one more than 200 miles north 

 of the Arctic Circle and the other more than 400 miles south of it. The 

 proposal to regard the present land-surface as a portion of the earth's 

 crust which has not only changed its form in the course of geological 

 history, but as a collection of slabs slowly drifting from place to place is 

 no new idea ; but we are indebted to Wegener for the development and 

 extension of a theory which in its present form has provided an incentive 

 to speculative minds and, it may be, a valuable clue to the solution of 

 diverse problems. Wegener speaks of the upper portion of the crust as 

 travelling in an easterly and westerly direction ; he also assumes a slight 

 movement of the poles. If it is permissible to postulate a drifting of 

 fractured slabs of the crust in a north and south direction, we can then think 

 of the disunited pieces, now occupying positions more or less remote from one 

 another, as the severed portions of a formerly compact region. To take 

 a concrete example : the Rhsetic plant beds of Eastern Greenland, now 

 remote from those of Sweden, may formerly have been portions of one 

 mass well to the south of the Arctic Circle. This may be merely a figment 

 of the imagination : on the other hand, some such expedient is almost 

 forced upon us if we are to find a solution to the problem presented by the 

 records of the rocks. There are, we are told, serious objections to Wegener's 

 hypothesis : it is at any rate true that the principle of drifting continents 

 has still to be proved tenable. But such evidence of correspondence, both 



