252 THE DISTRIBUTION AND CHARACTERS OF REPTILES. 



divided into isolated sections and as Gtilick states, " No two 

 portions of a species possess exactly the same average character," 

 so that such isolated sections must from the first be slightly dif- 

 ferent in average character. How that adaptation is brought 

 about I cannot pretend to discuss. If it be merely the result of 

 spontaneous internal impulses, the process of natural selection 

 will have to be invoked, in which case the cutting out of the less 

 adapted forms must accentuate the structural discontinuity : on 

 the other hand, it may be that compensatory impulses arise in 

 direct response to the environment. Secondly, structural dififer- 

 ences would probably tend to arise as a result of a different phy- 

 siology. 



The fact that some species can live without structural differ- 

 entiation in various diff'erent environments is not inconsistent 

 with the assuniiption that the groups belonging to the different 

 environments differ physiologically, though the differences may 

 be insufficient to eff'ect actual isolation. That physiological differ- 

 entiation should precede the morphological differences, is not 

 imprdbable when we consider that this is a well known pheno- 

 menon amongst Bacteria and other lowly organisms. 



Conclusions. — The study of the distinctive characters and dis- 

 tribution of closely allied species of reptiles in South Africa fur- 

 nishes the following data relating to the origin of specific discon- 

 tinuity : — 



1. That it is difficult to find evidence in favour of the muta- 



tion theory at any rate in the numerous cases of 

 closely related species which are geographically sepa- 

 rated. 



2. That the structural characters which separate species are 



sometimes adaptative, but often, perhaps usually, not 

 so. 



3. That closely allied species are often rigidl\' confined to 



adjacent but distinct areas, which dift'er in climatic 

 and vegetational conditions. 



From this last-mentioned fact we are probably entitled to 

 assume — 



(a) That such species must be physiologically adapted to 



the particular environment. 



(b) That they are therefore in a dual sense isolated from 



each other. Now any form of isolation will alone 

 imply some degree of discontinuity in structural 

 characters, since no two portions of the same stock 

 have exactly the same average character. 



(c) Such adaptation might conceivably be brought about 



by the cutting out of the less adapted (natural selec- 

 tion) in which process some structural discontinuity 

 would be caused. 



