AUSTRALOPITHECINES — ROBINSON 497 



Probably the chief variable is brain size, the early "Pithecanthropus" 

 forms being relatively small brained and hence with a slightly differ- 

 ently shaped braincase. But the known range in brain size of the 

 earliest hominines can be accommodated entirely within the observed 

 range of modern man. The dentition was also relatively robust in 

 the early forms, hence the face was fairly robust and prognathous. 

 But the dental and facial changes appear to be part of a continuous 

 sequence of modification of no great magnitude. There is no evi- 

 dence to show that there were several lines in which quite different 

 things were happening. From the amount of probable fragmenta- 

 tion of the early human population of the Old World, it is to be 

 expected that there were now and then, if not all the time, different 

 streams of evolution in which the changes were not identical or pro- 

 ceeding at the same rate. But what was happening in these various 

 lines of very low phyletic valence was essentially the same thing, so 

 that the characteristics of all the groups still overlapped to a very 

 marked degree. 



The nature of these differences and the lack of any significant di- 

 vergence of ecological requirements fit well into the picture of species 

 differences within a well-defuied genus among modern vertebrates. 

 Furthermore, it seems clear that once the hominines were well 

 launched on their path of cultural development, the character of 

 their evolutionary mechanism would have been modified. As Mayr 

 (1950) has pointed out, man occupies a wider range of environments 

 than any other animal. This would clearly seem to be a result of 

 his capacity for artificial adaptation. He can adapt himself to arctic 

 or to tropical conditions without significant change in his morphology 

 or physiology — in contrast to what occurs in other animals in such 

 cases. This is not to suggest that natural selection does not operate 

 on culture-bearing man — merely that its effects are modified by arti- 

 ficial adaptation. 



This capacity for artificial adaptation reduces his capacity to speci- 

 ate. Natural adaptation, mider the control of natural selection, to 

 different environmental conditions is the normal basis of speciation 

 and hence also of the achievement of greater levels of taxonomic 

 distinction. Eeduced rate of speciation and the more recent tendency 

 to interbreed increasingly over a wide area, have reduced very con- 

 siderably the possibilities of significant adaptive radiation within the 

 hominines. 



For these reasons, it seems to me that the hominines must all be 

 included witliin a single genus. Homo. This was suggested a decade 

 ago by Mayr (1950) as part of a taxonomic scheme which included 

 other aspects which do not appear to me to be valid. "Pithecanthro- 

 pus" should therefore be reduced simply to a species of Homo: H. 



