THE EVOLUTION OF HERBACEOUS PLANTS 303 



hypothesis which the Tertiary origin of the Andes and Himalayas 

 may perhaps be regarded as supporting) ; or that for some other 

 reason, possibly an increase in glaciation, the area of regions enjoy- 

 ing a cool climate in the tropics has at various times been consider- 

 ably augmented. 



In this connection it is of interest to note the evidence which is 

 accumulating as to the occurrence of more or less widespread 

 glaciation both at the transition from Cretaceous to Tertiary and 

 again during the Miocene. It is possible that the more ancient 

 herbaceous invasions were coincident with these earlier glacial 

 periods and that the later migration of such plants into the antip- 

 odes was contemporaneous with the extensive glaciation which 

 marked the latter part of the Tertiary. 



The theory put forward by HackeP and supported by Scharff^ 

 asserts that the boreal plants in antipodean South America are 

 extremely ancient types which have preserved their specific identity 

 for an exceedingly long time ; that they migrated over a land bridge 

 long since destroyed; and that instead of having come south along 

 the Andes they are at present going north. Such a theory is 

 certainly not tenable if herbs are proven to be of recent origin. 

 The increasing frequency of northern herbs as one goes south 

 along the Andes, the fact on which Hackel's hypothesis is based, 

 is well explained by a decrease in temperate areas within the tropics 

 as by a migration from the south. It is also very difficult to 

 imagine so many forms maintaining or almost maintaining their 

 specific identity for such a very long period of time. The present 

 theory of the origin of herbs certainly supports the' view that 

 they have entered the antipodes rather recently over land bridges 

 which have not had time to become fundamentally altered. 



We have already spoken of the antarctic continent as a highway 

 between South America and Australasia ; and the existence in com- 

 paratively recent times of such an extended Antarctica, able to 

 support a large fauna and flora, has been postulated by almost all 

 students of the distribution of antipodean plants and animals. 



' E. Hackel, "tJber die Beziehungen der Flora der Magellanslander zu jener d. 

 nordl. Europa und Amerika," Mitteil. naturw. Verein Steiermark (Botan. Sekt.) 

 (1905), PP- 110-15. 



^ R. F. Scharff, Distribution and Origin of Life in America (1912), p. 418. 



