Geometrica-Group of S.A. Tortoises. 207 



find them to-day is to be sought in germinal variation under the 

 selective influence of environmental changes. The study of their 

 geographical distribution shows that different types occupy different 

 areas, and there is some evidence that certain of the characters are 

 selective as regards these different environments. I am therefore 

 of opinion that the explanation of the present conditions is to be 

 found in germinal variations along determinate lines, influenced 

 by environmental forces. Given variation along definite lines, then 

 adjustment to environment is to my mind the key to the occurrence 

 of different forms in different districts. 



III. SUMMARY. 



1. Ten species of the geometrica-gvoup of South African 

 tortoises have been described by different systematists. In the present 

 paper attention has been directed, first, to an analysis of the variations 

 of the separate characters which are relied upon in describing the 

 species, and, second, to the associations and combinations of the 

 variations in the individual, involving the production of specific types. 



2. With certain exceptions, all the characters discussed are 

 present at one varietal stage or another in all the individuals. The 

 general uniformity of the characters is taken as proof of the homo- 

 geneity and recent differentiation of the geometnca-gvoup, and 

 evolution consists in the gradual elaboration of characters traceable 

 throughout the group. 



3. Certain characters are wanting or incipient at one extreme, 

 e.g., the femoral tubercle; certain new characters are restricted to 

 what may be considered as intermediate members of the group, e.g., 

 supplementary femoral shields, rows of caudal shields. These new 

 characters are to be regarded as but further expressions of tendencies 

 already indicated by other characters. 



4. All the characters present a gradual change in complexity 

 or elaboration, passing uninterruptedly from what we may consider 

 as one extreme to the other. They afford proof of Continuous 



or Determinate Variation (Orthogenesis). There is no evidence of any 

 rapid or sudden change from one condition to another, nor the sudden 

 introduction of new characters : Discontinuous or Indeterminate 

 Variation (Mutations). The transformation of any one character 

 may be in only one direction or in more than one. 



5. Certain characters are very fluctuating in some sub-groups, 

 and constant in others. The former condition may be taken as 

 indicative of the activity of the processes of germinal variation, 

 perhaps associated with environmental changes, and is conducive to 

 the establishment of new types ; the latter state as indicative of a 



cessation of germinal variation and constancy of environment, and 

 not conducive to the production of new forms. 



6. In the production of the specific type, it is found that the 

 stages reached in the transformation of different characters either 

 differ in the same individual (non-correlated variation) or vary pari 



