AUSTRALOPITHECINES — ROBINSON 499 



successfully for a long time. Both did in fact exist synchronously 

 in Africa over a substantial period of time, though at present they 

 are not known to have been sympatric. But Australopithecus and 

 Homo are much more closely related and are probably two phases 

 of the same phyletic sequence, though evidently one species of Aus- 

 tralopithecus was contemporaneous for a short time with an early 

 H. erectus in the Sterkfontein Valley. This overlap in time appears 

 to have been short; the ecological similarity between the two makes 

 it imlikely that it could have been long anywhere. But there is no 

 sharp discontmuity between Australopithecus and Homo — except in 

 brain size in the known specimens. But clearly there must have been 

 at least one line in which this gap also was bridged. So, while it is 

 clearly convenient to keep a generic distinction between the two 

 groups, it should be recognized that this is not a distinction of the 

 same type as that between Paranthropus and Australopithecus or 

 Paranthropus and Homo. This is just another of the numerous ex- 

 amples where the conventional Linnean taxonomic approach breaks 

 down. This "vertical" type of arbitrary taxonomic distinction — as 

 opposed to the "horizontal"' taxonomy of contemporaneous forms — 

 is one of the troublesome things which the paleontologist has learned 

 to live with but which does not trouble the neozoologist. 



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