248 INTRODUCTION TO EVOLUTION 



the names of known fossil forms. These are placed to represent successive 

 stages in human evolution, with the additional probability implied that 

 they may have contributed some of the genes possessed by their succes- 

 sors. Thus, the australopithecines may have contributed some genes to 

 Pithecanthropus, but so also may some other ancestors as yet unknown to 

 us (not all the lines leading to "Pithecanthropus" emanate from "Australo- 

 pithecines"). Similarly, Pithecanthropus is generally regarded as ancestral 

 to the mid-Pleistocene forms exemplified by the Swanscombe and Stein- 

 heim fossils, but it may or may not have been the only ancestor of these 

 forms. And, as we have noted above, these and later "transitional forms" 

 doubtless contributed both to the ancestry of Neanderthal man and to that 

 of Homo sapiens of modern type. 



The diagram does not indicate the extent of time during which each form 

 lived. The australopithecines, for example, may have arisen in the Plio- 

 cene, and they probably lived on as contemporaries of the earliest repre- 

 sentatives of Pithecanthropus. 



The Human Species 



As mentioned earlier, we regard all modern peoples as belonging to one 

 species, sapiens, of one genus. Homo. What is a species? In Chapter 14 we 

 shall discuss the attributes of this unit of classification. At present we shall 

 confine ourselves to a definition generally acceptable in the light of modern 

 biological knowledge, that of Mayr ( 1 942, 1 950 ) : "Species are groups of ac- 

 tually or potentially interbreeding natural populations, which are reproduc- 

 tively isolated from other such groups." Reproductive isolation is discussed 

 in Chapter 20 (pp. 471-473). Briefly, two populations are said to be repro- 

 ductively isolated from each other if they do not interbreed, and hence ex- 

 change genes, even when they have opportunity to do so. The red squirrels 

 and gray squirrels in our woods form an example of two species in one 

 genus; although they may live together in the same woods they do not inter- 

 breed. They are reproductively isolated from each other. 



What is the situation of the modern races of man? Each race is certainly 

 composed of actually or potentially interbreeding populations. Are the 

 races reproductively isolated from each other? Clearly there are no 

 anatomical or physiological incompatibilities separating them. As we shall 

 note later, however, reproductive isolation may at times have a psy- 

 chological basis. Observation seems to indicate some limited measure of 

 psychological isolation among human races, since members of one race 

 usually prejer to marry members of the same race rather than members 

 of other races. Exceptions are common, however. In the absence of social 



