The Theory of Evolution | 299 



be placed in the same genus, he could place the Java Man in the 

 genus Pithccantliropiis. Others might disagree with him but could 

 not demonstrate that he was wrong. Such disagreements rested only 

 on opinion. With the biological concept at the species level, things 

 were different; one had only to find out if two populations were ac- 

 tually interbreeding, or, in the absence of the physical contact 

 necessary to permit this gene exchange, were at least potentially 

 able to interbreed. Partially differentiated segregates within a spe- 

 cies were called subspecies and were considered, with considerable 

 justification, to represent the early stages of species formation. In 

 borderline cases, such as the Drosophila paiiJistorum example dis- 

 cussed in Chap. 10, where information derived from laboratory 

 crosses did not permit a clear "species or subspecies" decision, the 

 description "species in statu nascendi" has been used. 



In recent years a group of biologists has questioned the continu- 

 ing utility of the biological-species concept. They regard the ap- 

 parent distinctness of species (as a sweeping generality) as an 

 artifact of the procedures of taxonomy. These procedures decree a 

 hierarchic structure in which every entity to be recognized formally 

 with a "scientific" name must be assigned to some level in the 

 hierarchy. In other words, taxonomists were required to find distinct 

 entities, whether or not any existed. Soon after the idea of biological 

 species was promulgated, it became obvious that the great multi- 

 plicity of genetic systems found in plants, as discussed earlier, re- 

 sulted in a very limited applicability of the concept in botany. The 

 concept was ne\'er intended to apply to asexual organisms, but it 

 has also proved difficult to apply in the many groups of plants 

 where sexual reproduction makes up only one part of the genetic sys- 

 tem, where allopolyploidy has produced a reticulate phylogeny, or 

 where the "organism" is multiple ( as in lichens ) . As more knowledge 

 of the invertebrates is uncovered, the inadequacy of the biological- 

 species definition has also been apparent. 



The last stronghold of the utility of the concept was in the insects 

 and vertebrates. However, there seems to be only one entity, a 

 vertebrate, about which we have sufficient information to be reason- 

 ably safe in assuming that it is a "biological species." This verte- 

 brate is Homo sapiens. We have sufficient information about 

 interbreeding within the group to be relatively sure that all subdi- 

 visions can exchange genes with all others (the sort of information 

 that is usually lacking). In addition, man's nearest living relatives 

 are so different from him that the possibility of gene flow is dis- 

 counted. Recent investigations of insects, lohich did not start from 

 the premise that organisms must (in most instances) occur in dis- 

 tinct clusters, have indicated that the ease with which various groups 



