192 
PACIFIC SCIENCE, Vol. XX, April 1966 
Using the Indo-Malayan subregion as the ap- 
proximate center of the geographic range of 
this genus, the above distributional data indi- 
cate that the further one moves away from this 
center, in any direction, the more the addition 
of endemic species replaces the progressively de- 
pleted Indo-Malayan forms. Speciation, as the 
data indicate, has taken place on the periphery 
of the range, i.e., in those areas of comparative 
geographic isolation. 
Mayr (1942), Darlington (1948), and Brown 
(1957 and 1958) agree that geographic isola- 
tion plays a major role in the process of evolv- 
ing distinct species, although there is one great 
point of difference among the three. Darlington 
and Brown state that an adaptive change occurs 
in the center and spreads out to the periphery; 
then the population recedes, leaving some mem- 
bers isolated. Another change occurs in the cen- 
ter and spreads outward. If this second change 
is different enough from the previously isolated 
populations on the periphery, a new species is 
recognized. Mayr, however, postulates that "a 
new species develops if a population which has 
become geographically isolated from its par- 
ental species acquires, during the period of iso- 
lation, characters which promote or guarantee 
reproductive isolation when the external barriers 
break down.” 
The data presented here, showing the com- 
paratively large numbers of endemic species on 
the periphery of the range, support these mod- 
ern views on speciation in that geographic isola- 
tion plays a major role in the formation of 
distinct species. The data, however, do not 
favor either viewpoint as to where the adaptive 
changes occur. 
REFERENCES 
Brown, W. L., Jr. 1957. Centrifugal specia- 
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Cain, A. J., and G. A. Harrison. 1958. An 
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Cantwell, G. E. 1964. A revision of the genus 
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Darlington, P. J., Jr. 1948. The geograph- 
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