G. R. Wieland — Polar Climate in Time. 427 



of this exceedingly well represented bracliiopod genus extend- 

 ing quite through the Cambrian, Ordovician and Silurian, 

 that the fundamental difference between its earlier and later 

 history "is the presence of intermediate groups during the 

 former period, and their absence during the latter." In view 

 of his own studies and the testimony of others a summation of 

 the general facts is thus stated : — " Given a new and vigorous 

 stock in a favorable environment, the initiation of new species 

 takes place with great rapidity." 



It may of course be added that by any " rapid " evolution 

 I here mean only relatively rapid. Sudden changes of climate, 

 or the transportation from warm to much colder localities and 

 vice versa^ resnlt in very obvions changes in animals and plants, 

 but snch do not usually leave the stock a vigorous one, or end 

 in extinction. I have described an excellent example of this 

 kind in the case of the shutting out of salt water from the 

 Currituck Sound by the closing up of the Currituck Inlet in 

 North Carolina by drifting sands in 1828.^ Previous to that 

 year this inlet formed such a passage from the ocean through 

 a narrow onter beach into the waters of Currituck Sound as 

 is now formed by the new or Ocracock Inlet to Pamlico 

 Sound. With the closing of the Currituck Inlet there resulted 

 a conversion of upwards of one hundred square miles of shal- 

 low salt or brackish water to a fresh water area ; and it is within 

 the memory of men now living that the resultant changes in 

 the life of the sound were immediate and striking. Previ- 

 ously the sound had been a valuable oyster bed. But within 

 a few years after the exclusion of salt w^ater the oysters had 

 all died out, and their shells may now be seen in long heaps 

 where they have been thrown out in dredging for a boat way 

 in the shallow Coinjock Bay, a southwestern extension of the 

 sound. Furthermore, the salt water fishes were driven out, 

 and fresh water fishes took their places, whilst such changes 

 were produced in the vegetation as brought countless thou- 

 sands of ducks of species that had only been occasional before, 

 thus making this one of the finest hunting grounds on the 

 Atlantic Coast. 



Owing to the landlocked condition of the Arctic area, and 

 its numerous peninsulas and archipelagoes, such changes must 

 from the Mesozoic on often have occurred there and by depop- 

 ulation added to the general tendency to rapid change. 



Some idea of the amount of evolution which may be under- 

 gone in isolated and favorable areas after a general dispersion 

 has taken place, may be gained by the consideration of island 

 life. The great development of tree-like plants, elsewhere 

 * This Journal, p. 76, July, 1897. 



