Tin-. DISTKir.UTION AiXn CHAKACTKRS OF KKI'TILES. 249 



groups of species, we cannot be so certain. Rana fuscigula of 

 \\'estern Cape Colony has entirely webbed feet and shorter legs, 

 whereas Rana angolcnsis of Eastern Cape Colony has rather 

 longer legs, and the feet are described as three-fourths webbed, 

 but that these characters are peculiarly useful to each species in 

 its particular envdronment seems a little doubtful, especially as 

 their distribution areas slightly overlap and in intervening dis- 

 tricts both species may occur in the same locality. Yet, of the 

 geometric tortoises. Dr. Duerden wrote : 



Tf one were able to study the peculiarities of the environment closely, 

 there is little question that the variations would be found to be largely 

 adaptative. 



On the other hand, it is impossible to see utility in the struc- 

 tural characters which separate the cobras, the species being dis- 

 tinguished mainly through a slightl}' different arrangement of 

 the scales on the head, and such distinctions constitute the speci- 

 fic differences in many other closely allied species of snakes. 

 Lizards also present many cases of species differentiation based 

 on varied arrangements in the scutellation of the head or body. 

 Chama-elcon ventralis of Grahamstown is separated from C. 

 pumihis of Capetown by several characters, amongst the chief of 

 which is the shape of the lobes which form a fringe under the 

 throat. A(/ama hispida of the Cape division is distinguished 

 from distanti of the Free State and Transvaal, chiefly in that the 

 belly scales of the former are keeled and spinose, whilst those of 

 the latter are almost or entirely smooth. Similar differences 

 separate annata of Natal from acnlcata of the Kalahari and 

 Karroo. Bnfo regidaris. the common toad, has a granular belly, 

 whilst B. gariepcnsis, the Karroo species has a smooth belly. It 

 seems very probable indeed that the structural characters which 

 separate species are not necessarily of importance to the species, 

 or at any rate are not of life and death importance. Nevertheless 

 it is highly probable that in many cases there do exist between 

 allied species certain physiological differences which are adapta- 

 tive. There seems to me no other explanation of such facts as 

 the following : The common toad, Bitfo regularis, is wndely dis- 

 tributed throughout Africa (except Barbary), and is common in 

 all suitable localities from Egypt to the Cape, but does not occur 

 in the carroid parts of the Cape, where, however a closely-allied 

 species, B. gariepcnsis (granti) is fairly abundant. The Eg}^p- 

 tian cobra ranges from Palestine to Zululand, and the black- 

 necked cobra from Egypt and Senegambia to Natal, but neither 

 of them enters the Cape, where the yellow cobra is common. 

 Now seeing that the common toad enjoys such a wide distribution, 

 but does not penetrate into the Karroo, though it occurs on the 

 north, east and south sides thereof, we may fairly assume that it 

 cannot live there, and most probably also the Egyptian cobra 

 avoids Cape Colony because the climate does not suit its parti- 

 cular constitution. It seems to me. indeed, that the dift'erences 



