24 BULLETIN 186, UNITED STATES NATIONAL MUSEUM 



The dividing line between the two areas is the Khun Tan chain, 

 which in one case may be inhabited by a western race, in another by 

 an eastern race, and thus cannot be said definitely to belong to either 

 division ; however, since many eastern species not found at all farther 

 west extend their range to the western foot of these mountains (cf. 

 Cyanops faiostricta) , they have perhaps a stronger claim to be in- 

 cluded with the eastern provinces. 



The importance of the Khun Tan range as a zoogeographical boun- 

 dary between the Indo-Burmese and Indo-Chinese Provinces of the 

 Indo-Chinese Subregion is highlighted by the interesting fact that 

 in lowland species, distinct races of which occur right to its eastern 

 and western bases, we find no sign of intergradation between one form 

 and the other ; in short, an observer stationed at the crest of the divide 

 looks down to one side or the other upon territory inhabited by abso- 

 lutely typical individuals of whatever race. It is evident then that 

 neither division has been populated from the other, and since the Khun 

 Tan hills, for the most part of low elevation, would seem to offer no 

 important barrier to spread of populations, we must assume that these 

 cognate forms have, long after their ancestors left the mutual home- 

 land, here come face to face from diverse directions and found further 

 advance on the part of either precluded by prior occupation of its 

 ecological niche across the range. 



SUMMARY 



Northern Thailand may be separated into three faunal areas. The 

 majority of forms are common to all three and others are shared by 

 any two, but each has species and subspecies, whether resident or 

 migratory, peculiar to itself. 



The northern division has not only birds to be expected at its more 

 northern latitude but also a strong southern element that has traveled 

 up the valley of the Mae Khong. 



The western division has a characteristically Indo-Burmese fauna. 



The eastern division has a characteristically Indo-Chinese fauna. 



The Khun Tan mountain range is the natural boundary in this part 

 of Thailand between the Indo-Burmese and Indo-Chinese Provinces. 



VEGETATION 



The whole of our area lies well within the Tropics and is densely 

 clothed with vegetation to the summits of the highest mountains. 

 The various types of forest and other growth referred to throughout 

 this study are the following : 



1. Pa daeng ("red jungle"), an association covering perhaps half of 

 the forested portions of all Thailand north of the Isthmus of Kra. It 



