G. R. Wieland — Polar Climate in Time. 421 



graphic succession is learned in more detail, to be found over- 

 lapped and the more ancient. And, conversely, in the case of 

 two such remarkably similar florae as are found at essentially 

 the same latitude in the Lower Cretaceous of Portugal and 

 Maryland, there is a strong likelihood that they are really of 

 much the same age. 



Reasons fo?' the Origin of Prepotent Northern Stochs. — 

 As has been seen, the fact of extensive southern migration for 

 long periods of time is a patent one. It now remains to take 

 up the most interesting enquiry as to why hardy stocks origi- 

 nate at the north, and especially as to how their origin has 

 been governed by the secular decrease in temperature which 

 northern life shows to have taken place, as well as by the 

 shuttle of climatic change, caused by orbital eccentricity, 

 doubtless the most active of all factors producing change in 

 the life of the polar areas. It is trite to say that the enquiry 

 here briefly taken up must largely await future study, involv- 

 ing as it does the fundamental working principles of evolution. 

 But it is possible to mention certain salient elements of the 

 completer answer. 



Yegetal and animal forms are often flguratively spoken of as 

 driven south, or east, or west. Now forms of life may be 

 driven out of a region, that is exterminated within its bounds, 

 but unless rarely, in the case of the more intelligent higher 

 forms of animals fleeing from some newly appearing dreaded 

 enemy, they are never actually driven into a new habitat. 

 IS'or do they migrate except in a most general sense. They 

 simply make their way aided by or in spite of vicissitudes of 

 climate and season, as if from one side of a stream or one 

 hillside to another, changing more or less all thie while, while 

 somewhere to the rear the original stock is also changing into 

 a new one or being directly cut off. 



Again the conditions under which the life of a given locality 

 thrives are always in the long run retreating southward, and 

 other and hardier forms are always to be found to the north- 

 ward, though in less and less number as the secular approach 

 of glacial cold slowly and surely depopulated the Arctic area. 

 Now, only the hardy remnant of the once abundant northern 

 life yet dwelling on the borders of the great ice cap is left. 

 But it is a remnant with the power to invade, and that is still 

 pressing south in the same manner as the more profuse boreal 

 life of the past. Hence, as secular cooling is always going on, 

 and as the general nature of growth is constant, a condition of 

 high northern organic potential resulting in a southward stress 

 and movement must always exist. 



The idea here meant to be conveyed can be readily illus- 



