194 Report S.A.A. Advancement of Science. 



tentoria, intermediate in verreauxii, and to reach their climax in 

 oculifcra ; but, as regards the femoral tubercle, the climax is reached 

 in verreauxii, and the tendency diminishes in importance towards 

 tentoria on the one hand and oculifera on the other. 



Elsewhere {Records Albany Museum, Vol. I., p. 409) allusion 

 has been made to the fact that somewhat similar variations of the 

 femoral tubercle occur in the tortoises of the South African genus 

 Homofus. In the coast forms the spine is either wanting or very 

 feebly developed, while in those far inland on the high, dry plateaux 

 it is exceedingly well developed. We seem to have here another of 

 those striking instances of the influence of like environment producing 

 the same effect upon the members of two groups otherwise wholly 

 unrelated (Homeoplasty). 



The rows of enlarged circular shields found along the sides and 

 dorsal surface of the tail represent a new character, which is practic- 

 ally restricted to the verreauxii sub-group, and even here is not always 

 represented. They first appear, reach their climax, and as gradually 

 disappear within the same sub-group. Their development evidently 

 represents a response to the same tendency as that exhibited in the 

 femoral shields ; they probably protect this vulnerable part of the 

 body, and the two reach their highest development in the same 

 individuals. 



II. I. SPECIFIC TYPES. 



Hitherto attention has been directed to the individual characteris- 

 tics of the geometrica-'^xoxa^, their variations in the different sub- 

 groups, and mode of transformation from one stage to another. It 

 is manifest that the variations of any one character may serve as 

 a complete study. In nature, however, we find all the characters 

 associated in the individual, and it is the sum of these and other 

 features which constitutes the organism. Hence to complete the 

 study enquiry must be directed to the combination of the character- 

 istics making up the individual. 



As the various structures discussed are present at one develop- 

 mental stage or another in each individual member of the geometrica- 

 group, it is obvious that they offer great possibilities in the way of 

 combinations, unless, indeed, one character can be shown to vary 

 fari passu with all the others. Unless this latter condition is the I'ule, 

 one or more characters may have progressed or retrograded in com- 

 plexity while the remainder have continued unchanged, or all may 

 have changed in different degrees ; also a feature may be fixed and 

 stable in one group while fluctuating in others. The evolutionary 

 forces at work, whether internal or external, or both combined, 

 may influence some structures and not others, or some to a greater 

 degree than others, or even along different lines. 



It is conceivable that within a restricted series of organisms 

 like that of the geomeirica-gxoup, all the characters may vary 

 together, in which case there would be just as many specific types 

 produced as there are stages in the evolution of any one character. 



