632 Annals of the South African Museum. 



reproductive cells, particularly the ripe oospores, always allowing a 

 very wide margin within which variation may take place. 



5. Systematic Aspect. At present our knowledge of the behaviour 

 of the different forms of Volvox under varying conditions is still so 

 scanty that the existing classification can only be regarded as pro- 

 visional. Continuous observation in the field and repeated breeding 

 experiment as far as possible under natural conditions are badly 

 needed. 



Lacking a comparative study of the various forms in the living state 

 it seems best to accept existing species as described and to create 

 new species for new forms as they are discovered, particularly if 

 found in a different part of the world, giving at the same time as 

 fully illustrated and detailed a description as possible. 



But it is very difficult, if not impossible, to draw hard and fast 

 lines between closely allied " species." F. Rousseletii and F. capensis 

 provide a case in point. Both, particularly the former, are wide- 

 spread in Africa, and seem consistently distinct. Further, when 

 observed over long periods, as has been done for both species V. 

 capensis in its natural habitat of the Flats and V. Rousseletii in 

 Mr. Steer's Bietfontein cultures though very variable, they are 

 found to vary about a type which is distinct for the two. Yet the 

 differences are largely differences of degree, and when F. capensis 

 is found in warmer regions side by side with F. Rousseletii it un- 

 doubtedly approaches the latter more nearly, though still distinct. 

 Therefore, they must, for the present at any rate, be regarded as 

 distinct species. 



Again, on the one hand, F. capensis approaches very closely to 

 V. globator, while on the other F. Rousseletii seems almost identical 

 with Powers' (1908, p. 162) species F. perglobator from America, so 

 far as one can judge in the absence of certain descriptive details and 

 figures of the latter species. 



As regards the two former, whereas typical F. capensis is very 

 distinct from typical F. globator, yet the cold weather form of the 

 former approaches so much nearer to F. globator as to suggest the 

 possibility of the two being merely regional forms of the same species. 

 But without the necessary experimental evidence, or a detailed study 

 in the field of both members of the allied pairs, it is better at present, 

 for the sake of clearness if for no other reason, to retain the separate 

 specific rank. Possibly later, when the group is re-studied as a whole 

 and our knowledge of the various species has been extended by 

 experiment and study of living forms, some of the existing species 



