Geometrica-Group of S.A. Tortoises. 195 



Eut it is not probable that such regularity could be maintained. 

 Even under similar external forces one would expect, as a result 

 of germinal selection, that certain characters would be transformed 

 at a different rate in some forms than in others, or in different direc- 

 tions. Much more likely are we to have variations if we consider the 

 individuals placed under different environments, where the various 

 features will respond differently ; given time and isolation there seems 

 scarcely any limit to the possibility of the production of different 

 types, even with the comparatively few characters discussed. 



A consideration of the origin of even such a limited number of 

 forms as those constituting the geometrica-gxou^ opens up most of 

 the great questions involved in the evolution of the specific type, 

 and while a large number of the necessary facts are wanting, the 

 data available seem to throw some light upon the processes involved. 

 We have before us a group of organisms everywhere recognized as 

 made up of members very closely allied, and, presumably, having 

 a common genetic source, and many, with good reason, are already 

 recognised as distinct species. It therefore seems worthy of enquiry 

 as to how these specific types are constituted, and how they have 

 been evolved or transformed within the limits of the group. 



In any discussion of the nature of the specific type, the con- 

 clusions arrived at will largely depend upon the conception enter- 

 tained as to the definition of species. In the present connection, the 

 main point at issue is whether the extremes of a series of intergrading 

 forms are to be regarded as species, or only as sub-species or 

 varieties. The question will be more fully discussed later, but in the 

 following account I have accepted the latter conception, believing 

 that it more nearly represents the facts of nature, even if not 

 so satisfactory for systematic uniformity and orderliness. 



a. Testudo geometrica, Linneus, 1766. 



The typical forms included under this name have their charac- 

 ters very clearly defined, and among them are many representing 

 extreme modifications, when compared with those of other members 

 of the group. Among the specific distinctions are : (i) the 

 large, high shell, gradually sloping and narrowing in front ; (2) the 

 deep, downwardly directed, marginal shields, without or with only 

 a feeble margino-costal angulation ; (3) the long, narrow nuchal ; 

 (4) the five or six antebrachial shields, not overlapping. In all these 

 geometrica has diverged from the other sub-groups, but in its colora- 

 tion, both above and below, in the absence or rudimentary nature of 

 the femoral tubercle, and in the long beak, it does not differ 

 from certain of the other sub-groups. The shell attains a much 

 larger size than in any of the others, one specimen having a length of 

 24 cm. 



The combination of all the above characters makes geometrica 

 a well-defined species, were it not for the possible presence of transi- 

 tional types. Among the material available, these intermediate 

 types are wanting, and for the present the question of the isolation 



