Geometrica-Group of S.A. Tortoises. 203, 



way. I am convinced that if all the tortoises belonging to the 

 geometrica-group at present living in South Africa could be gathered 

 together no sharp distinction would be found between the extremes ; 

 there would be an almost imperceptible passage from one so-called 

 species to another ; there would be numerous specimens, of which 

 it would be impossible to say to which of two species they should 

 belong. 



In any discussion of specific types, the conclusions reached will 

 depend much upon the conception entertained of the term species. If 

 we consider a species to include a group of individuals having certain 

 distinct characters in common, and find that they breed true to 

 these characters, and continue to do so, then we must recognize 

 a large number of species within the geomeirica-group ; for almost 

 each locality has its group of individuals which differ from others 

 in features which might fairly be considered as constant. With 

 such slow breeding creatures as tortoises it would be difficult to prove 

 this experimentally, yet there can be little hesitation in holding that 

 such as oculifera, tentoria, verreauxii, trimeni, and geometrica would 

 breed true. 



When, however, we find that all the groups are connected with 

 one another by transitional forms, the above conception of a species 

 is insufficient. Probably the clearest expression upon the bearing of 

 transitional forms on taxonomy is that recently given by Ortmann 

 (Science, May nth, 1906) : "Of course, it is generally admitted that 

 species should breed true ; but this is also a necessary character 

 that belongs to the concept of variety. What distinguishes species 

 from varieties is the fact that a species is not connected by inter- 

 mediate or transitional forms with the most closely allied forms. 

 This latter principle is the one made use of exclusively (if possible) 

 by systematists, botanists as well as zoologists. In many cases, 

 indeed, it cannot be used on account of the insufficiency of our 

 knowledge ; but under such conditions new species are always 

 described with the tacit understanding that the demonstration of the 

 existence of intermediate forms will reduce them to the rank of 

 varieties." 



The theory of evolution by gradual modification implies that all 

 living organisms are genetically related, having had ancestors in 

 common. Were all these available for study there would be no such 

 assemblages as those involved in the concept of species. It is the 

 breaking of the chain, the extinction of intermediate forms, which 

 gives us species. The concept of species implies that a number of 

 individuals have sufficiently distinct and constant characters ta 

 separate them from their relatives, the transitional forms originally 

 connecting them and these nearest relatives being no longer existent. 



From this point of view, the results of the discussion upon the 

 taxonomic standing of the ten different species of the geometrica- 

 group may be summarized as follows : Both oculifera and geometrica 

 have numerous, well-defined, distinguishing characters, many of which 

 represent extreme variations of features transitional in others. 

 Regarded as combinations of characters, the two must for the present 



