Malaysian Phytogeography and Polynesian Flora 251 



of plants having native representatives in Malaysia are generally 

 distributed throughout Insulind, and the same is true of many 

 of the dominant genera. Yet certain families, such as the Diptero- 

 carpaceae,* that dominate the primary forests of western Malay- 

 sia, become distinctly unimportant in eastern Malaysia, and, 

 conversely, a considerable number of genera of the Myrtaceae 

 that occur in eastern Malaysia, fail to reach w^estern Malaysia. 

 Some of these genera are Eucalyptus, Mearnsia, Xanthostemon, 

 Osbornia, and Xanthomyrtus (the last two also are found in 

 Borneo). Furthermore, there is a remarkable development of 

 specific endemism; every large, medium-sized, and even small 

 island in the group has a very high percentage of species that, so 

 far as we know at present, are found only on that island, or on a 

 certain part of it, or on a restricted group of neighboring islands. 

 Had there not been some active limiting factor, there is no evi- 

 dent reason why a great many of these thousands of local species 

 should not now have more general geographic distribution, or 

 why certain families and genera should be dominant in or lim- 

 ited to specific parts of the Archipelago. This limiting factor 

 seems to be found in the geologic history of Malaysia. From the 

 Tertiary, a more or less continuous insular area existed in some 

 part of this region, the straits and arms of the sea inhibiting the 

 natural dispersal of a great many species of plants and animals. 

 We know little about the climatic conditions of Malaysia in 

 past geologic times, but we may deduce from the present-day 

 vegetation that for a long time the climate has not been strikingly 

 different from that which obtains today. Certainly there have 

 been no periods in which the low-altitude vegetation was affected 



* Merrill, E. D., 1923. Distribution of the Dipterocarpaceae. Origin and rela- 

 tionships of the Philippine flora and causes of the differences between the floras 

 of eastern and western Malaysia. Philip. Jour. Sci., 23:1-33, /. 1-8. 



