24- 



TH E DISTRIBUTION AND CHARACTERS OF REPTILES. 



common with the scheme of Zomirus and of the hispid Agcmias.*^ 

 The snakes of the genus Psammophis are also distributed in a simi- 

 lar manner. t In view of these facts, it may be stated that in South 

 Africa the environment directly or indirectly produces the species 

 as described later on, probably the simplest explanation would be 

 in terms of the natural selection theory with or without Lamarck- 

 ian factors, and yet it seems to me just possible that these dif- 

 ferent environments have merely operated in providing a means 

 of isolation for the several portions of the original varying stock. 

 The isolation afforded by the different environments prob- 

 ably is a real factor in the formation of species, and the fact that 

 allied species almost invariably occur in different environments 

 may mean that only in this way can isolation be effected: still it 

 is hardly likely that this is the only way in which environment 

 affects the species. There are, moreover, several classes of facts 



l' K. X ». jl 1«. X 



Distribution of Agama hispida and allies. 



which are not easily explained in accordance with the view of 

 geographical isolation as a prime or sole factor. Firstly, there 

 are some cases known where the most closely related species 

 live in the same localities : as instances may be mentioned the 

 frogs Rana oxyrhynchus and R. mascarenicnsis or the grass 

 frogs Rana grayi and R. fasciain, and amongst snakes Boodon 

 lincatus, B. infernalis and B. guttatus or Chlorophis natalensis 

 and C. hoplogaster. In these cases, though their whole distri- 

 bution areas may not completely coincide, the related species 

 often occur together in the same place, and there can be no reason 

 for suspecting that they ever were geographically separated into 

 distinct areas. It may be argued indeed that even here each 

 species may be topographically isolated from its nearest ally 

 through some peculiar mode of life, such, for example, as obtains 



* See Duerden in Report S.A.A. Advancement of Science, Kimberley 

 (1906), 3, 204. 



t See Records Albany Museum 2 [4] 268. 



