Geometrica-Group of S.A. Tortoises. 179 



the central areola. The original type, Testit3,o geometrica, is a well- 

 known Linnean species, found in the Cape District. 



With the extended exploration of South Africa, more and more 

 of the geo7netrica-\\ke^ tortoises have been discovered, so that at the 

 present time the group comprises ten described species : — i, Testudo 

 geometrica, Linneus, 1766; 2, T. ociilifera, Kuhl, 1820; 3, T. ten- 

 toria, Bell, 1828; 4, T. verreauxii, Smith, 1839; 5, T. trimeni, 

 Boulenger, 1886; 6, T. smithii, Boulenger, 1886; 7, T. fiskii, 

 Boulenger, 1886; 8, T. strauchi, Lidth de Jeude, 1893; 9, T. sei- 

 mundi, Boulenger, 1903; 10, T. boettgeri, Siebenrock, 1904. It is 

 a significant fact that five of the species have been founded on single 

 specimens, and in not more than two or three instances has a large 

 number of specimens been available for determining the limits c f 

 variation or the relationship of the species to others previously 

 described. 



The fact that so many species of a clearly defined grou-p occur 

 in one region, all acknowledged to be very closely allied, suggested 

 that a thorough study of the geometrica-torioises on the spot might 

 yield results of importance as regards the origin of the variations, 

 their degree of distinctness, and their relationships ; in other words, 

 might illustrate in some way the method of evolution of the different 

 species. For the prosecution of such a study large numbers of speci- 

 mens are required, obtained from as many different sources as 

 possible. From efforts already made, about 300 examples have 

 been procured from various localities in South Africa. The collec- 

 tion can probably be regarded as fairly representative of the different 

 types of the geometrica-^xou'^), though a perfect study of this kind 

 would require that every individual specimen, living and dead, should 

 be compared, and, where possible, the results stated in statistical 

 terms. For specimens received acknowledgments are due to the 

 Director of the South African Museum, Cape Town, of the Natal 

 Government Museum, and of the King William's Town Museum, as 

 well as to numerous contributors throughout South Africa, among 

 whom A'Tr. S. C. Cronwright Schreiner, M.L.A., deserves special 

 mention. 



During the progress of the work, as more and more specimens 

 became available for study, the more difficult became the task of 

 arranging them systematically among the recognized species, and the 

 more evidence was accumulated as to the close relationship of the 

 species so-called, thereby confirming the idea of their common genetic 

 nature. Moreover, it soon became manifest that the different series 

 include some remarkable transitional forms, and also that characters 

 incipient or fluctuating in one group of specimens were well developed 

 or fixed in other groups. In every direction evidence was afforded of 

 determinate variation or variation along definite lines (orthogenesis) 

 in contrast with indeterminate or discontinuous variation (mutation). 



Though the occurrence of transitional series of organisms is by 

 no means unfamiliar to the zoologist, yet it is verv desirable that they 

 should all be thoroughly worked out with the object of discovering 

 what particular new light they may shed upon the all -important 



