Annals of the Transvaal Museum. 
71 
It will be seen from the above facts that the two areas are by no 
means homogeneous throughout their length, inasmuch as the tropical 
fauna from the north gradually loses in intensity as it passes southwards, 
whilst a somewhat opposite state of affairs obtains with the peculiar South 
African fauna. The rivers of South Africa, no doubt, act as efficient 
barriers ag ainst the dispersal of a small proportion of the species. On the 
west side* the Orange River seems to be the southern boundary of a 
number of tropical species which enter the western sub-region, and, 
consequently, in western Cape Colony the peculiar fauna of South Africa 
is relatively most strongly developed. On the east side the Limpopo 
River is apparently an impassable barrier to such typical South African 
lizards as Mctbuia trivittata , Pseudocordylus microlepidotus , and 
Chamaesaura aenea ; and, on the other hand, such tropical species as 
Agama kirki are not to be found south of that river. 
It may be possible to further divide the western region, and, perhaps, 
also the eastern, into a number of zones according to altitude, but there is 
not sufficient data to determine this. It is known, however, that there are 
certain species (. Zonurus cordylus and Mabuia homalocephcila) which 
occur only on the coastal strip of South Africa. 
In conclusion, we may summarize as follows : — The South African 
zoological region is sub-divided into western and eastern sub-regions with 
the limits as above defined, but the separation, is not a very profound one, 
for some species and many genera are common to both sub-regions, and, 
though some genera and many species are absolutely confined to one of the 
regions only, this cannot be said of any natural group of genera nor of 
any family of lizards, whilst, lastly, the segregation is largely in accordance 
with different environmental conditions, seeing that the most 
characteristic elements of the western sub-region show many structural 
adaptations for a deserticolous habitat. 
