548 DEVELOPMENT OF THE N.O. MYRTACEjE, 



The answer to these, is practically insoluble without a knowledge 

 of the geographical factor. The study of the geological record 

 undoubtedly suggests that the earth has passed through various 

 periods of genial and almost cosmopolitan climates, and that these 

 have alternated with periods of marked differential climate. The 

 cosmopolitan and genial climates have tended to produce cosmo- 

 politan, or at least widely-spread, floras whenever sea-barriers 

 have not been opposed to distribution. On the other hand, the 

 variation of climates has tended to floral differentiations. 



The Cretaceous was a period of such marked tendency to genial 

 climate, and the fertile tropical flora appears to have possessed a 

 wide range in that time. On the other hand, xerophytic and de- 

 pauperate types are almost wholly wanting in the collections 

 obtained from the Cretaceous and earlier Tertiary deposits. 



The Pliocene and later periods have presented marked differen- 

 tiations of climate, culminating in the Pleistocene glaciation. This 

 would tend to produce marked local variations in the floras (and 

 faunas), and thus an erstwhile, widely-spread type would, upon 

 a gradual differentiation of climate, tend to contract its range alto- 

 gether as regards the primitive type, and to bring it nearer the 

 equator; while local floras would arise as the result of adaptation 

 to new environments under conditions of isolation. 



One would, therefore, expect the primitive types of Myrtaceae 

 to have had a great tropical range in the Cretaceous, probably 

 extending into the regions now temperate in both Hemispheres, 

 and later, upon the great Post-Cretaceous changes of climate, to 

 have been confined to the tropics, and to have become locally dif- 

 ferentiated in such places as America and Australia, where they 

 were not opposed by such severe competition as in the Northern 

 Hemisphere. 



Differentiation of Myrtace^. 



The Eugenias and and the Myrtles appear to be the genera near- 

 est in morphological characters to the earlier types of the family. 

 In the wide belt of the fertile tropics during Cretaceous time, where 

 large deserts and high land-barriers were very rare, there was a 

 tendency for the Myrtaceae to become cosmopolitan in range, and 



